[DanielDent]

You send server-to-server messages on port 587? That is... non-standard. But an interesting approach.

You are using your users' gmail credentials? Or are you simply delivering over 587 the same as you would over 25?

[jlgaddis]

It's very "non-standard" -- that's not what the submission port (587/TCP) -- not "subscription port" -- is for.