When they started charging for Mandrill, I switched to MailGun. The last time I checked, it lets you send 12,000 messages per month for free.
There's a MailGunX class in the Notify extra that works in MODX. IIRC, it comes with a bunch of baggage, but it works quite well.
Last time I checked mchimpx does still work with v3. I could be wrong and have my timescales mixed up but pretty sure I had a project last year where I used this and had a problem but it was user error.
The api is pretty well documented though it shouldn't be too hard to create a custom hook if you have the knowledge how.
