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When sending a test email on an account that is not correctly configured yet (for this particular case, the DNS MX entry is not pointing yet to Google), the following error raises:
Notice: Undefined offset: 0 at SmtpMailSystem->mail() (line 169 of /path/to/drupal/sites/all/modules/contrib/smtp/smtp.mail.inc).
Adding a small check before entering the switch statement fixes the issue. Patch attached in the following comment.