Postmark is a premium solution for emails that can boast an extremely high, fast delivery rate. There is built in email verification, and trouble-shooting solutions if the need arises.
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Pro Messages are unlikely to be marked as spam
Postmark is designed to send transactional messages. It specifically warns users not to use it to send weekly newsletter -- this way the emails sent by Postmark are less likely to be marked as spam, increasing the likelihood of your messages reaching the users.
Pro Detailed analytics
Postmark offers very detailed analytics for your emails. For starters, it stores 45 days worth of email logs where you can search to find out which emails were sent, received, who received them, who read them and who marked them as spam.
There are also recipient profiles which keep track of everyone who gets your emails, complete with a list of messages that they have opened, deleted or marked as spam.
Pro Multiple server locations
Postmark has 5 server locations around the world (3 in the US, 1 in Amsterdam and 1 in SIngapore). This way emails will be sent fast regardless of where your users are.
Pro Messages are Saved for 45 Days
All outgoing messages are saved for 45 days so you can see what you're sending to who. This is important if you are sending any dynamic content in your emails.
Postmark are one of the most expensive (if not the most expensive) services in the market, at $1.5 per 1000 emails.