Thanks for taking the time to comment. It’s great to finally get our problems confirmed and explained. The only thing that I’m sure that we are unable to upload are ‘bounced messages.’ That is something we see quite frequently.

We’re well aware of the APPEND issue, and have a possible fix rolling out soon. We do fail messages we can’t parse, which is technically against the RFC. We also sometimes fail due to load issues, which is why you haven’t found a correlation. The new version should at least differentiate between the two errors, and will hopefully do a better job of handling the load failures. Ultimately, adding a message in gmail is expensive, and was always done async, and we’re working against it.

What to do with messages we can’t parse is more challenging. Some servers use “default” headers and make the message the body. If you do figure out something we can’t parse that we should, let us know. My email is in here, but failing that you can post to the gmail help forums, if you have enough information our support folks who monitor it should know to raise it with us.