After finding my bounce messages were seemingly disappearing into the ether when sending mail with mail(), I'm trying to configure CiviCRM to send using G Suite's SMTP relay.
I've configured the SMTP settings in Administer > System Settings > Outbound E-mail and the test button on that screen sends an e-mail that is successfully delivered, seemingly correctly through G Suite as indicated by this line in the e-mail headers:
Received: from example.net ([192.0.2.1]) by smtp-relay.gmail.com with ESMTP id g185sm57996wmg.25.2020.05.20.09.26.24 for <firstname.lastname@example.org>; Wed, 20 May 2020 09:26:24 -0700 (PDT)
When I try and send an actual mailing using CiviMail, though, I get a 100% bounce rate, all with a bounce of type 'syntax'. In the bounces report I can see bounce reasons given in the format:
Failed to set sender: email@example.com [SMTP: Invalid response code received from SMTP server while sending email. This is often caused by a misconfiguration in Outbound Email settings. Please verify the settings at
That message is obviously truncated. If I log in to the bounce handling account there don't seem to be any messages there that might show me the full error, either in the inbox, or in INBOX/CiviMail/ignored or INBOX/CiviMail/processed. Is there any way to see the rest of the message so I can diagnose what's going on?