enewsletter bounced back due to message is not RFC 5322 compliant

Hi,

i have created a newsletter using the enewsletter plugin (like i've done before) and scheduled it for a distribution.
the following issues happens:
1. the schedule job didnt execute. i had to remove cron job and send manually.
2. the manual job does not complete automatically and i have to continue the send manually.
3. even though my list is considered to be clean i get some bounce back in my bounced mail box with the following message "SMTP error from remote server for TEXT command, host: mx1.hotmail.com (65.55.33.135) reason: 550 5.7.0 (COL004-MC6F3) Message could not be delivered. Please ensure the
message is RFC 5322 compliant."
4. when i try to continue the send job manually i get an error message to check my outgoing mail settings which i did. i was successful to send test message to test the settings...

please advise how the above can be resolved.
especially #3

Thanks,

David