Hi all this morning I found that google block by default application that try send email without match newer security standard. I always use SecureSMTPClientSession without problem since few days ago. There's a walk around in gmail configuration for let "less secure" application to send email.
After few debugging I think the problem was related to the context build with CLIENT_USE instead for example TLSV1_2_CLIENT_USE...but this didn't help.
I think the problem is related to the authentication method because google suggest to use OAuth 2.0 and I think that is the problem. Have you found the same behavior?