Mail to some addresses not sent - Postfix - Connection refused

Hello,
I am having this strange problem. Emails can be sent to some addresses but not others.

OS: CentOS Linux 7.6.1810 (Core)
Product: Plesk Onyx 17.8.11 Update #95

/var/log/maillog says this:-
Mar 21 17:13:54 server416 postfix/pipe[1981]: BD72B21DB1: to=<user@domain.com>, relay=spamassassin, delay=8.1, delays=0.17/0.01/0/7.9, dsn=2.0.0, status=sent (delivered via spamassassin service)

Mar 21 17:13:54 server416 postfix/qmgr[438]: BD72B21DB1: removed

Mar 21 17:13:54...

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