Due to an unexpected service interruption, I've recently had to evaluate numerous options for SMTP services, to replace a one that was showing signs of performance degradation. The AWS SES is an obvious choice, but what if it is not available, or your organization requires diversifying to other providers? Below is a quick list of contenders that I have compiled, with my comments on each. 

The requirement for me is that the successful SMTP service has to integrate with a number of applications for email sending: drupal, wordpress, and ruby on rails. It should provide me with a username + password to be able to send emails via SMTP protocol. This bar for success is low: there are many services that provide this capability. Below is a short list.

mailtrap.io

1000 emails/mo is free.

$15/mo for 10,000 emails/mo sounds reasonable.

Their service works well, as far as I can tell.

Postal

A complicated self-hosted solution. To use it, you will need to configer DNS records, a dockerized app, and databases, to name a few presequisites. This solution allows you to own your own sending capability entirely.

mailcow.email

A complicated self-hosted solution. To use it, you will need to configer DNS records, a dockerized app, and databases, to name a few presequisites. This solution allows you to own your own sending capability entirely.

smtp.com

At $25/mo for 50,000 emails/mo, they are definitely a contender to AWS SES.

sendgrid.com - degraded service