Ask YC: What do you use for outbound email sending?

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?