Is there a signature being attached to the emails that are rejected? If so remove the signature and see if the emails go through. It seems that items that some users add to their signature trigger spam filters on providers servers. If it works without the signature then you need to figure out what the server does not like about the signature.

Yes it worked! Thank you Airmail!

viviana010 said

My employer uses your email service and when he tries to send a message, this comes up "An error occurred while sending mail. The mail server responded: 5.7.1 [P4] Message blocked due to spam content in the message.. Please check the message and try again." We do not see the problem with the message and have even retyped it. But for some reason Thunderbird is finding fault with it. Also we have adjusted the port number as posted with previous questions and that did not do anything. Thanks!

Note that Thunderbird is a email client on your computer for desktop Windows. Mac OSX and Linux and is not a web service.