Login to Webmail fails, and the following error is shown in the mailog:
Nov 10 11:56:46 _hostname_ imapd: IMAP connect from @ [127.0.0.1]No such user'[email@example.com](mailto:firstname.lastname@example.org)' in mail authorization database
Nov 10 11:56:51 _hostname_ imapd: LOGIN FAILED, ip=[127.0.0.1]
Cause & Resolution
The above "No such user" error message is generic and may as well appear due to other causes, e.g. when such mailbox does not exist in the domain. Thus, please check that it indeed exists and that all configuration files are in place.
The issue may also happen if your server contains mailboxes with capital letters in their names. Check this article for more information:
113752 Email addresses containing capital letters do not work
Workaround: If it is indeed the case, you may try logging in to Webmail using the case-sensitive mailbox name. You should be able to look it up both in the control panel and in command line.