Merge pull request #4028 from Daniel15/patch-2

Enable maildir_very_dirty_syncs by default

data/conf/dovecot/dovecot.conf

@@ -24,6 +24,11 @@ mail_plugins = </etc/dovecot/mail_plugins
 mail_attachment_fs = crypt:set_prefix=mail_crypt_global:posix:
 mail_attachment_dir = /var/attachments
 mail_attachment_min_size = 128k
+# Significantly speeds up very large mailboxes, but is only safe to enable if
+# you do not manually modify the files in the `cur` directories in
+# mailcowdockerized_vmail-vol-1.
+# https://docs.mailcow.email/manual-guides/Dovecot/u_e-dovecot-performance/
+maildir_very_dirty_syncs = yes
 
 # Dovecot 2.2
 #ssl_protocols = !SSLv3