We recently migrated the hosting and email account of a user to another server. We've started receiving this error message when trying to send messages. 

# Start of Message #

Your message did not reach some or all of the intended recipients. 

Subject: test
Sent: 4/7/2015

The following recipient(s) cannot be reached:

550 5.7.1 *some code* This message does not comply with required standards.

# End of Message # --
Obviously I edited a few stuff there in the message, such as the recipient email, but the error is as is - Code: 550 5.7.1

The POP and SMTP settings of the user still route through the same "mail.domain.tld" address, using the same ports. SMTP Authentication is enabled. 

I found this on the microsoft support site: 
https://social.technet.microsoft.com/Forums/en-US/585c1a77-83ca-4a42-8de8-1fd34bd9a9d1/your-message-did-not-reach-some-or-all-of-the-intended-recipients?forum=officeitpro

But it doesn't really identify a common workaround. Any ideas on how to fix this issue?

Hi,

Does this happen with any messages that you've sent? What about a simple plain text email message?

As far as I know, 550 5.7.1 signifies an "Unable to relay" error. The specific error "This message does not comply with required standards." is usually caused by a content scanner on the SMTP server.

For example, if you are trying to send an email message which contains hyperlinks in the body, and the SMTP server doesn't allow hyperlinks, then you will probably get the same error message.

I would suggest you have a check on this, or contact your ISP to help with the check.

Regards,

Ethan Hua
TechNet Community Support