SMTP error from remote mail server after RCPT TO: 451

Hello

I've noticed that some emails that we send are not delivered. The same recipient get's email from two persons in the office when another person sends an email it's not being delivered with this message:
SMTP error from remote mail server after RCPT TO: 451

I've checked and it's actually fine when sending an email from web client, but it fails when sending from Outlook 2016 with IMAP/SMTP connection.

Any ideas what might be causing this issue?

Thanks

 

Thanks for your reply.

I set up all the Outlooks manually. Another user tried to email that particular person and it failed with the same message - that was from different PC and Windows Live Mail.

I can see it works now for someone that it didn't work before, I think the only thing I've done was disabling a Bitdefender Anti-Ransomware tool - I didn't think that would affect emails - I'll do some tests and post an update.