Issue Emails sent from outsorced app return host error when sent to same domain name they came from

[Shwele]

I have a client who has Plesk Onyx and they gave me their credentials to check for issue they are having but cant seem to spot it.
I don't know Plesk version due that account I have doens't have access to tools&settings.

We have connected their mailboxes via SMTP and IMAP/POP3 to CiviCRM website they are using, for the purpose of sending newsletters.

Emails are sent alright vie newsletter feature, but when they are sending emails to the same domain name (their colleagues for tests, ex. from [email protected] to [email protected]) they get bounced with HOST syntax error. anyone knows what might be causing issues?

Thanks in advance to all!