Gmail & Comcast SMTP

I manage a custom domain for my company which is hosted thru our Comcast servers. One of my users has all of his mail (incoming and outgoing) through his Gmail account. Recently his outgoing messages started bouncing, reported it, and I went to work attempting to verify his SMTP settings. This is where I'm perplexed. 

When accessing his account through a separate mail client (in this case Apple Mail) I'm able to get the account working properly in all ways -- the SMTP settings are set to port 465, SSL enabled, the credentials correct; it all works like a charm. 

However in plugging those same settings into his Gmail account, I run into a block. He's having no trouble receiving mail from this account, which is currently being accessed by POP3 settings.

But the SMTP settings won't resolve properly within the Gmail account -- again the credentials match those in my standalone client, the port is 465 and SSL is checked, but after clicking the "add account" button within the Gmail interface, I receive a red error message reading

I've exhausted everything I can think of in trying to resolve this situation, and can't figure out around how these SMTP settings can be fully functional within a standalone client but not with the Gmail interface. 

I've managed a workaround solution for him, in which his Gmail account has the "reply-to" address set to our custom domain, and that should be fine for the short term, but I'd really like to fix this problem.  

Has anyone else run into this issue?