services: Add 'imap4d-service-type'.

* gnu/services/mail.scm (<imap4d-configuration>): New record type.
(imap4d-shepherd-service): New procedure.
(%default-imap4d-config-file, imap4d-service-type): New variables.
* gnu/services/mail.scm (Mail Services): Document it.

doc/guix.texi

@@ -16559,6 +16559,36 @@ the @code{operating-system}'s @code{user-accounts} in order to deliver
 the @code{postmaster} mail to @code{bob} (which subsequently would
 deliver mail to @code{bob@@example.com} and @code{bob@@example2.com}).
 
+@subsubheading GNU Mailutils IMAP4 Daemon
+@cindex GNU Mailutils IMAP4 Daemon
+
+@deffn {Scheme Variable} imap4d-service-type
+This is the type of the GNU Mailutils IMAP4 Daemon (@pxref{(mailutils)
+imap4d}), whose value should be an @code{imap4d-configuration} object as in
+this example:
+
+@example
+(service imap4d-service-type
+         (imap4d-configuration
+           (config-file (local-file "imap4d.conf"))))
+@end example
+@end deffn
+
+@deftp {Data Type} imap4d-configuration
+Data type representing the configuration of @command{imap4d}.
+
+@table @asis
+@item @code{package} (default: @code{mailutils})
+The package that provides @command{imap4d}.
+
+@item @code{config-file} (default: @code{%default-imap4d-config-file})
+File-like object of the configuration file to use, by default it will listen
+on the tcp port 143 of @code{localhost}.  @xref{(mailutils) Conf-imap4d} for
+details.
+
+@end table
+@end deftp
+
 @node Messaging Services
 @subsection Messaging Services
 

gnu/services/mail.scm

@@ -64,7 +64,12 @@
             exim-configuration
             exim-configuration?
             exim-service-type
-            %default-exim-config-file))
+            %default-exim-config-file
+
+            imap4d-configuration
+            imap4d-configuration?
+            imap4d-service-type
+            %defualt-imap4d-config-file))
 
 ;;; Commentary:
 ;;;
@@ -1776,3 +1781,41 @@ exim_group = exim
           (service-extension activation-service-type exim-activation)
           (service-extension profile-service-type exim-profile)
           (service-extension mail-aliases-service-type (const '()))))))
+
+
+;;;
+;;; GNU Mailutils IMAP4 Daemon.
+;;;
+
+(define %default-imap4d-config-file
+  (plain-file "imap4d.conf" "server localhost {};\n"))
+
+(define-record-type* <imap4d-configuration>
+  imap4d-configuration make-imap4d-configuration imap4d-configuration?
+  (package     imap4d-configuration-package
+               (default mailutils))
+  (config-file imap4d-configuration-config-file
+               (default %default-imap4d-config-file)))
+
+(define imap4d-shepherd-service
+  (match-lambda
+    (($ <imap4d-configuration> package config-file)
+     (list (shepherd-service
+            (provision '(imap4d))
+            (requirement '(networking syslogd))
+            (documentation "Run the imap4d daemon.")
+            (start (let ((imap4d (file-append package "/sbin/imap4d")))
+                     #~(make-forkexec-constructor
+                        (list #$imap4d "--daemon" "--foreground"
+                              "--config-file" #$config-file))))
+            (stop #~(make-kill-destructor)))))))
+
+(define imap4d-service-type
+  (service-type
+   (name 'imap4d)
+   (description
+    "Run the GNU @command{imap4d} to serve e-mail messages through IMAP.")
+   (extensions
+    (list (service-extension
+           shepherd-root-service-type imap4d-shepherd-service)))
+   (default-value (imap4d-configuration))))