Sending email doesn't work (roundcube and SMTP)

[NothernPeak]

I purchased a new VPS with Plesk 12 a month ago.

Now I have a problem - I can't send emails in Mozilla Thunderbird or in Roundcube (SMTP error 250).

Probably it happened after the recent auto-upgrade (12.0.18 Update #13.)

I've read carefully this forum and I discovered that many people with Plesk 12 have exactly same problem.

I have another VPS with Plesk 11.5 and there are no problems with emails at all.