SMTP Module affecting commenting

This is strange; how can the SMTP module affect someone leaving comments on the site?  I had an issue with the SMTP module that was related to the mail host and not the module.  During that time, I was getting data not accepted errors.  This morning I had a member leave a support ticket:

"SMTP Error: Data not accepted. 219328". Everytime I try to post anything I get this message. If I refresh my page, it will post, but not unless I refresh the page.

So why would a non working SMTP module affect someone posting on the site?

It looks like email is sent when you make particular post, in this case it can affect.

Thanks Alex, I think this is related to a third party module that sends out emails.