I try to send mail using Mathematica to my Gmail account using this solution and it works correctly.
The mail was sent correctly and I got this message:
{"MessageID" -> "15416335454959396639.4.WolframLanguage.xxxxxxx@gmail.com"}
However, even though I can see the message in my Sent list, but I did not receive the mail, even after a long time! What is the problem?