Mail error in log

Hello there,
is it possible to list somehow e.g. via occ command mailboxes appended to nextcloud instance?

I am getting a log error: OCA\Mail\Exception\ServiceException: IMAP error synchronizing account -id-: Error connecting to mail server. And in error details then Horde_Imap_Client_Exception: Error connecting to mail server. Always twice, for -id- 1 and 2. But I can’t get any info what address calls the error. The instance has 30 users, I am wondering why just id 1 and 2 call the error.

I think the ID 1 is the nextcloud admin but I can’t see any problems with admin emails - it seem to be OK - admin emails are sending and being received normally.

IMAP server is a common czech free email hosting provider

Thanks for a piece of advice.

Regards, Ondrej