Starting around 11 April Gmail started to mess with the TLS. If in the “send as” settings you had anything in the MX hostname, that does not match what’s going on after your @ and your cert, your mail server got a reject, even if his origin matches the certificate. I had ‘mx.andreybondarenko.com’, had to change to ‘andreybondarenko.com’. I didn’t had touched this settings for ages. Discussion by the link below.