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Why some single Email messages fail on some @gmail.com addresses??? Puzzling

Hello everyone,
Thanks in advance for your help on this strange one. One of my classes is sending Messaging.SingleEmailMessage using a VF Email template to Contact. I am facing a stange behavoir. In all cases, the email is created correctly, and showd as fired, based on the debug-log.
BUT when the contact email is a '@gmail.com' address, the email is - I guess - not sent. It is not received for sure. Change the contact email to a '@yahoo.com' or something else and it all works fine.
Other classes using the same contact with similar code but a different VF email template and the '@gmail.com' addresses send the email as expected. ?????????????????????
Any idea? Anyone had faced such a strange pattern? Thanks in advance for your help,