SMTP SMTP AUTH OAUTH2 RFC4954 RFC6749 # # Server-side AUTH XOAUTH2 REPLY AUTH 334 XOAUTH2 supported REPLY dXNlcj11c2VyAWF1dGg9QmVhcmVyIG1GXzkuQjVmLTQuMUpxTQEB 235 Authenticated # # Client-side smtp SMTP OAuth 2.0 authentication mail body smtp://%HOSTIP:%SMTPPORT/908 --mail-rcpt recipient@example.com --mail-from sender@example.com -u user --oauth2-bearer mF_9.B5f-4.1JqM -T - # # Verify data after the test has been "shot" EHLO 908 AUTH XOAUTH2 dXNlcj11c2VyAWF1dGg9QmVhcmVyIG1GXzkuQjVmLTQuMUpxTQEB MAIL FROM: RCPT TO: DATA QUIT mail body .