Hello Aldevinas,
it seems the errors are coming as a response to the mass sent. Your recipients' inbox server is sending to your SMTP messages indicating it's blocking your content. The reasons could be many: badly formed messages, recipient server filters above sender reputation, or easy to be mistaken, wrong domain address in the SMTP server.
Apparently, it's not a software problem, and I wasn't been able to replicate in any way.

Are you sure about the distribution list content? and what about your SMTP reputation? You can easily check it using web tools.
It could be useful if you would show the received message header informing you about the errors.

Best Regards
Giuseppe
HansaWorld Support