Unable to send an email from Plesk mailbox to a certain external domain:
postfix/qmgr: 334625FDB0: from=<firstname.lastname@example.org>, size=619, nrcpt=1 (queue active)
dovecot: imap-login: Login: user=<email@example.com>, method=DIGEST-MD5, rip=::1, lip=::1, mpid=26010, secured, session=<8Sp/TnhR38MAAAAAAAAAAAAAAAAAAAAB>
postfix/smtp: 334625FDB0: to=<firstname.lastname@example.org>, relay=mail.domain.tld[203.0.113.2]:25, delay=5.6, delays=0.22/0.01/2/3.4, dsn=5.0.0, status=bounced (host mail.domain.tld[203.0.113.2] said: 554 Spam Filter Active  (in reply to end of DATA command))
The following message gets delivered to the sender as a bounce-back:
This is the mail system at host plesk.example.com.
I'm sorry to have to inform you that your message could not be delivered to one or more recipients. It's attached below.
For further assistance, please send mail to postmaster.
If you do so, please include this problem report. You can delete your own text from the attached returned message.
Emails sent to all other external servers get routed to Spam folder.
Server IP is blacklisted globally and spam settings on a recipient server are too strict.
To get email delivered to email@example.com, mail.domain.tld spam settings should be adjusted