I need to send out automated emails to users (e.g. invitations, share notices, etc.) and want to guarantee that recipients don't receive them in their spam folder.
I can minimize by utilizing SPF/DK/DKIM but it's not a guarantee. Services like Port25 that specialize in outbound MTA is quite expensive.
What do you guys use/suggest?
Spammers ruined email and it can now be pretty hard to pass some spam filters. Of course, 95% of your emails will make it through if you do it yourself, but the last few % can be hard to get.
I recently started sending through Fastmail.fm for our website and it's been working fine. I simply setup sSmtp on our servers and everything that is sent through Sendmail is now relayed through Fastmail.
Another company I've considered is authsmtp.com. I haven't tried them, but their pricing seems to be very reasonable. The reason I didn't go with them is that I already had a Fastmail account (and I hate paying twice for the same thing ;-) )
We did Gmail before, but setuping Postfix to relay through them can be a major pain (mainly because of the way they authenticate). Fastmail (and I assume authsmtp) was a no-brainer.
Another thing that will help delivery tremendously is SPF. Make sure to set it properly (with hard fail if possible, ie: -all).
Good luck!