In the InvisionPowerBoard (IPB), i have found error:

Email Error Log Manager

This server does not support authorisation
503 503 AUTH command used when not advertised

what fixed this was to go to Admin area / System / System settings / Email Set-up and switch "Enhanced SMTP HELO" from EHLO to HELO or back. Then in Users section / Validating, tick the user and resend the e-mail. Now worked. Stats & Logs ú Logs / E-mail error log no longer added new error entry.