check the logs:

i can highly recommend a colorful log output:

yum install ccze; # rpm based
apt install ccze; # apt based

and this script that will „follow“ all changes to all logfiles on your system…

vim /scripts/mon_all_logs.sh 
#!/bin/bash

find /var/log/ -type f \( -name "*" \) ! -path '*.gz*' -exec tail -n0 -f "$file" {} + | ccze

so you get EVERY error message that happens on the system… now the error seems to be in this log file:

# black and white
tail -f -n 50 /var/log/dovecot.log
# in color
tail -f -n 50 /var/log/dovecot.log | ccze

error message:

Aug 14 10:14:34 imap(user@domain.com): Error: user user@domain.com: Initialization failed: namespace configuration error: inbox=yes namespace missing 
Aug 14 10:14:34 imap(user@domain.com): Error: Invalid user settings. Refer to server log for more information. 

solution/howto fix:

as seen on: https://forum.vestacp.com/viewtopic.php?t=13395

all you have to do is add this little yellow line at the right place, and restart:

systemctl restart dovecot

systemctl restart exim

vim /etc/dovecot/conf.d/15-mailboxes.conf

##
## Mailbox definitions
##

# NOTE: Assumes "namespace inbox" has been defined in 10-mail.conf.
namespace inbox {
  inbox = yes # added this one
  #mailbox name {
    # auto=create will automatically create this mailbox.
    # auto=subscribe will both create and subscribe to the mailbox.
    #auto = no

    # Space separated list of IMAP SPECIAL-USE attributes as specified by
    # RFC 6154: \All \Archive \Drafts \Flagged \Junk \Sent \Trash
    #special_use =
  #}

  # These mailboxes are widely used and could perhaps be created automatically:
  mailbox Drafts {
    special_use = \Drafts
  }
  mailbox Junk {
    special_use = \Junk
  }
  mailbox Trash {
    special_use = \Trash
  }

  # For \Sent mailboxes there are two widely used names. We'll mark both of
  # them as \Sent. User typically deletes one of them if duplicates are created.
  mailbox Sent {
    special_use = \Sent
  }
  mailbox "Sent Messages" {
    special_use = \Sent
  }

  # If you have a virtual "All messages" mailbox:
  #mailbox virtual/All {
  #  special_use = \All
  #}

  # If you have a virtual "Flagged" mailbox:
  #mailbox virtual/Flagged {
  #  special_use = \Flagged
  #}
}
admin