Sparkpost service no longer free - any recommendations?
Two weeks ago I would suggest Mailgun but the plugin supplied for webhook settings seems to be updated to work for new CiviCRM versions.

But if you somehow make it work (and so integrate the service with Civi successfully) the service itself is quite successful for most of the mail providers.

Here comes the last two weeks... Yahoo (and AOL) domains keep rejecting the newsletter I sent because I was sharing an IP with some bad players. People at Mailgun are quite helpful and respond very quickly. They've changed the IP assigned to our account and the mails were accepted, but in another bulk sending, we had the problem all over again.

So I was tired of trying out providers and set up a postfix myself, I'm sending my mails with my own server. It needs maintenance and a bit experience to set it up but I think it was worth the effort. I can compare better in few months when new mailings will be sent.

The ironic part is, Yahoo has a program for bulk mailers and I had to enroll to that in order to have some positive reputation for my server and it was not based on IP address. So maybe I could try to add my domain to the program while I was using Mailgun and it could change something but I was already done a lot in server side, so this worked for me...

Hope it helps...