I'm posting this here in case anyone has the same problem as I did. I browsed the forums and found a few threads but nothing that solved my problem. So I'm posting my own solution for other people to reference.

This issue happened with b2evolution 5.0.6-stable. The email settings were all correct and the comments and message form worked fine on a test server. Upon moving to the client's host server, the message form broke and gave the above error; comments worked fine.

The issue turned out to be two-fold: first, the user email that was receiving the form did use gmail. However, the address was set to 'activated'. I tried changing the address to use an email for the client's domain name, which reset the 'activated' to 'email changed'.

My fix turned out to be really obvious: open up phpMyadmin, edit the user settings directly and change the email address to 'auto-activated'. This solved the problem immediately.

Had I known this, I wouldn't have frantically sent a detailed email to the web host for support as the forums suggested this was an issue with the server settings. So here I leave my solution in the hopes that it saves another poor soul from pulling their hair out.