>>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.