I don't want to be the party pooper, but without knowing too many details I can tell you that you're in for a bad time. You're essentially asking for abuse, as spammers will use your service to forward their junk, and whoever is hosting your service is going to receive abuse reports and act on them.
I've worked at a web hosting company before - not a major operation at a time, but we did enough business to staff one floor of an office building with our support and management people.
Something like 25%-50% of our support volume was spammers and scammers trying to sneak under the radar and get us to restore their service after we'd blacklisted them. A significant portion of these were using stolen credit cards.
Yes, sadly you are correct. I will do my best to monitor outbound SMTP frequency and blatant spam abuse, although so far it has been surprisingly pleasant. Thank you for the announcing this concern.
This is precisely why we don't provide free account at FastMail any more - and even then the free trials are causing us trials and tribulations right now.
Honestly, good luck - but you're going to find that giving away something for free to other people is very different than running it for yourself right about the first time something goes wrong and a few thousand very angry people want to know what's up with their email.
(and as an aside, I really think good email service is worth paying for, and having been woken in the middle of the night to fix things that customers never even noticed - I know that it won't stay surprisingly pleasant forever)
Because of the domain registration costs, using emailing with Pawnmail is not free. This should dramatically reduce the incentive of using Pawnmail to spam.
Btw, @vortico I'm using Pawnmail for my new domain name and it looks quite nice so far, thanks!
Fastmail is the only good option outside of running your own email. Glad to be a customer as I'm currently fed up with fighting spam. Also, you can't beat the Fastmail web interface.
Yeah, but it still seems like a great service. This... versus the $5/mo to pay to DigitalOcean to host essentially nothing other than postfix & cyrus daemons... is quite reasonable. Please don't let the Spectre of Spammers discourage you from this very appealing old-school sort of venture...
There are mean people on the internet :(