diff --git a/src/comp/cli.rs b/src/comp/cli.rs
deleted file mode 100644
index a64a331..0000000
--- a/src/comp/cli.rs
+++ /dev/null
@@ -1,29 +0,0 @@
-use anyhow::Result;
-use clap::{self, App, Arg, ArgMatches, Shell, SubCommand};
-use log::debug;
-use std::io;
-
-pub fn subcmds<'s>() -> Vec<App<'s, 's>> {
-    vec![SubCommand::with_name("completion")
-        .about("Generates the completion script for the given shell")
-        .args(&[Arg::with_name("shell")
-            .possible_values(&["bash", "zsh", "fish"])
-            .required(true)])]
-}
-
-pub fn matches<'a>(app: fn() -> App<'a, 'a>, matches: &ArgMatches) -> Result<bool> {
-    if let Some(matches) = matches.subcommand_matches("completion") {
-        debug!("completion command matched");
-        let shell = match matches.value_of("shell").unwrap() {
-            "fish" => Shell::Fish,
-            "zsh" => Shell::Zsh,
-            "bash" | _ => Shell::Bash,
-        };
-        debug!("shell: {}", shell);
-        app().gen_completions_to("himalaya", shell, &mut io::stdout());
-        return Ok(true);
-    };
-
-    debug!("nothing matched");
-    Ok(false)
-}
diff --git a/src/comp/mod.rs b/src/comp/mod.rs
deleted file mode 100644
index 4f77372..0000000
--- a/src/comp/mod.rs
+++ /dev/null
@@ -1 +0,0 @@
-pub mod cli;
diff --git a/src/compl/arg.rs b/src/compl/arg.rs
new file mode 100644
index 0000000..d123f93
--- /dev/null
+++ b/src/compl/arg.rs
@@ -0,0 +1,34 @@
+//! Module related to completion arguments.
+//!
+//! This module provides subcommands and an argument matcher for the completion command.
+
+use anyhow::Result;
+use clap::{self, App, Arg, ArgMatches, Shell, SubCommand};
+use log::debug;
+
+/// Subcommands related to the completion generation.
+pub fn subcmds<'a>() -> Vec<App<'a, 'a>> {
+    vec![SubCommand::with_name("completion")
+        .about("Generates the completion script for the given shell")
+        .args(&[Arg::with_name("shell")
+            .possible_values(&Shell::variants()[..])
+            .required(true)])]
+}
+
+/// Enumeration of all possible completion matches.
+pub enum Match<'a> {
+    /// Generate completion script for the given shell slice.
+    Generate(&'a str),
+}
+
+/// Completion arg matcher.
+pub fn matches<'a>(m: &'a ArgMatches) -> Result<Option<Match<'a>>> {
+    if let Some(m) = m.subcommand_matches("completion") {
+        debug!("completion command matched");
+        let shell = m.value_of("shell").unwrap();
+        debug!("shell: {}", shell);
+        return Ok(Some(Match::Generate(shell)));
+    };
+
+    Ok(None)
+}
diff --git a/src/compl/handler.rs b/src/compl/handler.rs
new file mode 100644
index 0000000..5fb0efc
--- /dev/null
+++ b/src/compl/handler.rs
@@ -0,0 +1,16 @@
+//! Module related to completion handling.
+//!
+//! This module gathers all completion actions triggered by the CLI.
+
+use anyhow::{anyhow, Context, Result};
+use clap::{App, Shell};
+use std::{io, str::FromStr};
+
+/// Generate completion script from the given [`clap::App`] for the given shell slice.
+pub fn generate<'a>(shell: &'a str, mut app: App<'a, 'a>) -> Result<()> {
+    let shell = Shell::from_str(shell)
+        .map_err(|err| anyhow!(err))
+        .context("cannot parse shell")?;
+    app.gen_completions_to("himalaya", shell, &mut io::stdout());
+    Ok(())
+}
diff --git a/src/compl/mod.rs b/src/compl/mod.rs
new file mode 100644
index 0000000..a333e6c
--- /dev/null
+++ b/src/compl/mod.rs
@@ -0,0 +1,9 @@
+//! Module related to shell completion.
+//!
+//! This module allows users to generate autocompletion scripts for their shells. You can see the
+//! list of available shells directly on the [clap's docs.rs website].
+//!
+//! [clap's docs.rs website]: https://docs.rs/clap/2.33.3/clap/enum.Shell.html
+
+pub mod arg;
+pub mod handler;
diff --git a/src/imap/cli.rs b/src/domain/imap/cli.rs
similarity index 95%
rename from src/imap/cli.rs
rename to src/domain/imap/cli.rs
index e05d538..dce1092 100644
--- a/src/imap/cli.rs
+++ b/src/domain/imap/cli.rs
@@ -2,7 +2,7 @@ use anyhow::Result;
 use clap;
 use log::debug;
 
-use crate::domain::{config::entity::Config, imap::ImapServiceInterface};
+use crate::domain::{config::entity::Config, imap::service::ImapServiceInterface};
 
 pub fn subcmds<'a>() -> Vec<clap::App<'a, 'a>> {
     vec![
diff --git a/src/domain/imap/mod.rs b/src/domain/imap/mod.rs
new file mode 100644
index 0000000..f41d052
--- /dev/null
+++ b/src/domain/imap/mod.rs
@@ -0,0 +1,4 @@
+//! Modules related to IMAP.
+
+pub mod cli;
+pub mod service;
diff --git a/src/domain/imap.rs b/src/domain/imap/service.rs
similarity index 99%
rename from src/domain/imap.rs
rename to src/domain/imap/service.rs
index 093fc69..0b12de7 100644
--- a/src/domain/imap.rs
+++ b/src/domain/imap/service.rs
@@ -4,9 +4,10 @@ use log::{debug, trace};
 use native_tls::{self, TlsConnector, TlsStream};
 use std::{collections::HashSet, convert::TryFrom, iter::FromIterator, net::TcpStream};
 
-use crate::{domain::account::entity::Account, flag::model::Flags, msg::model::Msg};
-
-use super::config::entity::Config;
+use crate::{
+    domain::{account::entity::Account, config::entity::Config, msg::entity::Msg},
+    flag::model::Flags,
+};
 
 type ImapSession = imap::Session<TlsStream<TcpStream>>;
 type ImapMsgs = imap::types::ZeroCopy<Vec<imap::types::Fetch>>;
diff --git a/src/mbox/cli.rs b/src/domain/mbox/cli.rs
similarity index 94%
rename from src/mbox/cli.rs
rename to src/domain/mbox/cli.rs
index 30355fd..90cd0a3 100644
--- a/src/mbox/cli.rs
+++ b/src/domain/mbox/cli.rs
@@ -3,8 +3,7 @@ use clap;
 use log::{debug, trace};
 
 use crate::{
-    domain::imap::ImapServiceInterface,
-    mbox::model::Mboxes,
+    domain::{imap::service::ImapServiceInterface, mbox::entity::Mboxes},
     output::service::{OutputService, OutputServiceInterface},
 };
 
diff --git a/src/mbox/model.rs b/src/domain/mbox/entity.rs
similarity index 100%
rename from src/mbox/model.rs
rename to src/domain/mbox/entity.rs
diff --git a/src/domain/mbox/mod.rs b/src/domain/mbox/mod.rs
new file mode 100644
index 0000000..e85c41a
--- /dev/null
+++ b/src/domain/mbox/mod.rs
@@ -0,0 +1,2 @@
+pub mod cli;
+pub mod entity;
diff --git a/src/domain/mod.rs b/src/domain/mod.rs
index d690368..c3446e0 100644
--- a/src/domain/mod.rs
+++ b/src/domain/mod.rs
@@ -3,4 +3,6 @@
 pub mod account;
 pub mod config;
 pub mod imap;
+pub mod mbox;
+pub mod msg;
 pub mod smtp;
diff --git a/src/msg/attachment.rs b/src/domain/msg/attachment.rs
similarity index 100%
rename from src/msg/attachment.rs
rename to src/domain/msg/attachment.rs
diff --git a/src/msg/body.rs b/src/domain/msg/body.rs
similarity index 100%
rename from src/msg/body.rs
rename to src/domain/msg/body.rs
diff --git a/src/msg/cli.rs b/src/domain/msg/cli.rs
similarity index 99%
rename from src/msg/cli.rs
rename to src/domain/msg/cli.rs
index aa93092..4d4b714 100644
--- a/src/msg/cli.rs
+++ b/src/domain/msg/cli.rs
@@ -15,14 +15,16 @@ use url::Url;
 
 use super::{
     body::Body,
+    entity::{Msg, MsgSerialized, Msgs},
     headers::Headers,
-    model::{Msg, MsgSerialized, Msgs},
 };
 use crate::{
-    domain::{account::entity::Account, imap::ImapServiceInterface, smtp::*},
+    domain::{
+        account::entity::Account, imap::service::ImapServiceInterface, mbox::cli::mbox_target_arg,
+        smtp::service::SmtpServiceInterface,
+    },
     flag::model::Flags,
     input,
-    mbox::cli::mbox_target_arg,
     output::service::{OutputService, OutputServiceInterface},
 };
 
diff --git a/src/msg/model.rs b/src/domain/msg/entity.rs
similarity index 100%
rename from src/msg/model.rs
rename to src/domain/msg/entity.rs
diff --git a/src/msg/headers.rs b/src/domain/msg/headers.rs
similarity index 100%
rename from src/msg/headers.rs
rename to src/domain/msg/headers.rs
diff --git a/src/msg/mod.rs b/src/domain/msg/mod.rs
similarity index 98%
rename from src/msg/mod.rs
rename to src/domain/msg/mod.rs
index 7c77f6f..ac0abf9 100644
--- a/src/msg/mod.rs
+++ b/src/domain/msg/mod.rs
@@ -22,7 +22,7 @@ pub mod cli;
 
 /// Here are the two **main structs** of this module: `Msg` and `Msgs` which
 /// represent a *Mail* or *multiple Mails* in this crate.
-pub mod model;
+pub mod entity;
 
 /// This module is used in the `Msg` struct, which should represent an
 /// attachment of a msg.
diff --git a/src/domain/smtp/mod.rs b/src/domain/smtp/mod.rs
new file mode 100644
index 0000000..81d6738
--- /dev/null
+++ b/src/domain/smtp/mod.rs
@@ -0,0 +1,3 @@
+//! Modules related to SMTP.
+
+pub mod service;
diff --git a/src/domain/smtp.rs b/src/domain/smtp/service.rs
similarity index 100%
rename from src/domain/smtp.rs
rename to src/domain/smtp/service.rs
diff --git a/src/flag/cli.rs b/src/flag/cli.rs
index 2573d34..c475d94 100644
--- a/src/flag/cli.rs
+++ b/src/flag/cli.rs
@@ -2,7 +2,10 @@ use anyhow::Result;
 use clap;
 use log::debug;
 
-use crate::{domain::imap::ImapServiceInterface, flag::model::Flags, msg::cli::uid_arg};
+use crate::{
+    domain::{imap::service::ImapServiceInterface, msg::cli::uid_arg},
+    flag::model::Flags,
+};
 
 fn flags_arg<'a>() -> clap::Arg<'a, 'a> {
     clap::Arg::with_name("flags")
diff --git a/src/imap/mod.rs b/src/imap/mod.rs
deleted file mode 100644
index 4f77372..0000000
--- a/src/imap/mod.rs
+++ /dev/null
@@ -1 +0,0 @@
-pub mod cli;
diff --git a/src/lib.rs b/src/lib.rs
index 92033d2..43da26b 100644
--- a/src/lib.rs
+++ b/src/lib.rs
@@ -12,9 +12,7 @@
 //!
 //! [here]: https://doc.rust-lang.org/book/ch07-02-defining-modules-to-control-scope-and-privacy.html
 
-/// `comp` stands for `completion`. This module makes it possible to create autocompletion-settings
-/// for himalaya for your shell :)
-pub mod comp;
+pub mod compl;
 
 /// Everything which is related to the config files. For example the structure of your config file.
 pub mod config;
@@ -23,19 +21,10 @@ pub mod config;
 /// read-flag.
 pub mod flag;
 
-/// A wrapper for creating connections easier to the IMAP-Servers.
-pub mod imap;
-
 /// Handles the input-interaction with the user. For example if you want to edit the body of your
 /// message, his module takes care of the draft and calls your ~(neo)vim~ your favourite editor.
 pub mod input;
 
-/// Everything which is related to mboxes, for example creating or deleting some.
-pub mod mbox;
-
-/// Includes everything related to a message. This means: Body, Headers, Attachments, etc.
-pub mod msg;
-
 /// Handles the output. For example the JSON and HTML output.
 pub mod output;
 
diff --git a/src/main.rs b/src/main.rs
index 918395f..c8b23e3 100644
--- a/src/main.rs
+++ b/src/main.rs
@@ -5,16 +5,20 @@ use log::{debug, trace};
 use std::{convert::TryFrom, env, path::PathBuf};
 
 use himalaya::{
-    comp,
+    compl,
     config::cli::config_args,
     domain::{
-        account::entity::Account, config::entity::Config, imap::ImapService, smtp::SmtpService,
+        account::entity::Account,
+        config::entity::Config,
+        imap::{self, service::ImapService},
+        mbox, msg,
+        smtp::service::SmtpService,
     },
-    flag, imap, mbox, msg,
+    flag,
     output::{cli::output_args, service::OutputService},
 };
 
-fn parse_args<'a>() -> clap::App<'a, 'a> {
+fn create_app<'a>() -> clap::App<'a, 'a> {
     clap::App::new(env!("CARGO_PKG_NAME"))
         .version(env!("CARGO_PKG_VERSION"))
         .about(env!("CARGO_PKG_DESCRIPTION"))
@@ -27,7 +31,7 @@ fn parse_args<'a>() -> clap::App<'a, 'a> {
         .subcommands(imap::cli::subcmds())
         .subcommands(mbox::cli::subcmds())
         .subcommands(msg::cli::subcmds())
-        .subcommands(comp::cli::subcmds())
+        .subcommands(compl::arg::subcmds())
 }
 
 fn main() -> Result<()> {
@@ -50,23 +54,23 @@ fn main() -> Result<()> {
     //     return Ok(msg_matches_mailto(&app, &url, smtp)?);
     // }
 
-    let args = parse_args();
-    let arg_matches = args.get_matches();
+    let app = create_app();
+    let m = app.get_matches();
 
-    // Check completion before init config
-    if comp::cli::matches(parse_args, &arg_matches)? {
+    // Check shell completion BEFORE any entity or service initialization.
+    if let Some(compl::arg::Match::Generate(shell)) = compl::arg::matches(&m)? {
+        let app = create_app();
+        compl::handler::generate(shell, app)?;
         return Ok(());
     }
 
-    debug!("init output service");
-    let output = OutputService::new(arg_matches.value_of("output").unwrap());
-    debug!("output service: {:?}", output);
+    let output = OutputService::new(m.value_of("output").unwrap())?;
 
     debug!("init mbox");
-    let mbox = arg_matches.value_of("mailbox").unwrap();
+    let mbox = m.value_of("mailbox").unwrap();
     debug!("mbox: {}", mbox);
 
-    let config_path: PathBuf = arg_matches
+    let config_path: PathBuf = m
         .value_of("config")
         .map(|s| s.into())
         .unwrap_or(Config::path()?);
@@ -74,7 +78,7 @@ fn main() -> Result<()> {
     let config = Config::try_from(config_path.clone())?;
     trace!("{:#?}", config);
 
-    let account_name = arg_matches.value_of("account");
+    let account_name = m.value_of("account");
     debug!("init account `{}`", account_name.unwrap_or("default"));
     let account = Account::try_from((&config, account_name))?;
     trace!("{:#?}", account);
@@ -86,10 +90,10 @@ fn main() -> Result<()> {
     let mut smtp = SmtpService::new(&account)?;
 
     debug!("begin matching");
-    let _matched = mbox::cli::matches(&arg_matches, &output, &mut imap)?
-        || flag::cli::matches(&arg_matches, &mut imap)?
-        || imap::cli::matches(&arg_matches, &config, &mut imap)?
-        || msg::cli::matches(&arg_matches, mbox, &account, &output, &mut imap, &mut smtp)?;
+    let _matched = mbox::cli::matches(&m, &output, &mut imap)?
+        || flag::cli::matches(&m, &mut imap)?
+        || imap::cli::matches(&m, &config, &mut imap)?
+        || msg::cli::matches(&m, mbox, &account, &output, &mut imap, &mut smtp)?;
 
     Ok(())
 }
diff --git a/src/mbox/mod.rs b/src/mbox/mod.rs
deleted file mode 100644
index 9225677..0000000
--- a/src/mbox/mod.rs
+++ /dev/null
@@ -1,2 +0,0 @@
-pub mod cli;
-pub mod model;
diff --git a/src/output/service.rs b/src/output/service.rs
index 786f811..21c88b0 100644
--- a/src/output/service.rs
+++ b/src/output/service.rs
@@ -1,6 +1,7 @@
-use anyhow::Result;
+use anyhow::{anyhow, Error, Result};
+use log::debug;
 use serde::Serialize;
-use std::fmt;
+use std::{fmt, str::FromStr};
 
 #[derive(Debug, PartialEq)]
 pub enum OutputFmt {
@@ -8,11 +9,13 @@ pub enum OutputFmt {
     Json,
 }
 
-impl From<&str> for OutputFmt {
-    fn from(slice: &str) -> Self {
+impl FromStr for OutputFmt {
+    type Err = Error;
+    fn from_str(slice: &str) -> Result<Self, Self::Err> {
         match slice {
-            "json" => Self::Json,
-            "plain" | _ => Self::Plain,
+            "JSON" | _ if slice.eq_ignore_ascii_case("json") => Ok(Self::Json),
+            "PLAIN" | _ if slice.eq_ignore_ascii_case("plain") => Ok(Self::Plain),
+            _ => Err(anyhow!("cannot parse output `{}`", slice)),
         }
     }
 }
@@ -21,7 +24,7 @@ impl fmt::Display for OutputFmt {
     fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
         let slice = match self {
             &OutputFmt::Json => "JSON",
-            &OutputFmt::Plain => "Plain",
+            &OutputFmt::Plain => "PLAIN",
         };
         write!(f, "{}", slice)
     }
@@ -51,8 +54,11 @@ pub struct OutputService {
 
 impl OutputService {
     /// Create a new output-handler by setting the given formatting style.
-    pub fn new(fmt: &str) -> Self {
-        Self { fmt: fmt.into() }
+    pub fn new(slice: &str) -> Result<Self> {
+        debug!("init output service");
+        debug!("output: {}", slice);
+        let fmt = OutputFmt::from_str(slice)?;
+        Ok(Self { fmt })
     }
 
     /// Returns true, if the formatting should be plaintext.