This can be mitigated by limiting the amount of data that is transmitted with a single 3DES session key. I doubt that Gmail accepts that many gigabytes of data

This can be mitigated by limiting the amount of data that is transmitted with a single 3DES session key. I doubt that Gmail accepts that many gigabytes of data in a single SMTP session anyway, so I doubt it's impacted by this flaw. (AES-GCM requires rekeying, too, also at intervals that exceeds typical SMTP session lengths, and is not considered terminally broken because if this.)