[WhatsName]

I'm afraid this will soon be abused by spammers & criminals around the world. I think having a free plan in the space of sending mail is a bad idea and puts paying customers at risk of being disrupted because your IP (-block) landed on someones blacklist.

[slig]

OP's service, as far as I understand, is a MITM proxy. The send emails using your credentials, not theirs.

[WhatsName]

So it can still act as a proxy for stolen Gmail, Outlook credentials or brute forcing passwords?