>>>In this case I am using my gmail account and smtp server to send the messages. It works fine from my PC, but fails on the client's PC. Not knowing enough about this, does that fit into what you described?
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>>Which port are you using to send your email to gmail server (normally 587 or 465)? You have to provide credentials to connect to gmail. Are they good?
>
>Yes, I am using the exact same credentials on my PC successfully.

Standard SMTP outgoing mail is on port 25. Most ISPs require traffic on that port to go to their SMTP server or smart host.

If GMail is using a different port, then that can get around that restriction. But, you may have to confirm with your client:

- that outgoing traffic on that non-standard port is allowed through their firewall
- that their ISP is not doing anything to block traffic on that port (that would be unusual but should still be checked if other more obvious possibilities don't pan out)

I don't know if you're trying to get this to work for just one client, or if you're trying to create something more general-purpose. If the former, by all means do what you need to do to get it to work. If the latter, it's probably best to try to work with port 25, and specify the SMTP server name and credentials (if required) as needed.