Error message: Ran into problems sending Mail. Error: 500 5.5.0 Line too long

[Plantje]

When sending the newsletters I get a lot of errors: "Ran into problems sending Mail. Error: 500 5.5.0 Line too long". I don't think for all users this error is returned. I am among the affected users. As long as I don't empty the queue I don't get any E-mails anymore.

This seems to be an issue with the SMTP server. Can I view a preview of this newsletter somewhere? See what the culprit may be?

Other suggestions on how to solve this?