Why separate transactional email from marketing?

These are two kinds of mail with two different requirements, and mixing them creates unnecessary risk.

Transactional email

Order confirmations, password resets, invoices, notifications. The recipient expects them and they must arrive immediately.

Marketing email

Newsletters and campaigns. The recipient is not waiting for them, and a slight delay does not matter.

Why separate them

A campaign that generates complaints lowers your reputation. If your confirmations go through the same channel, they go down with it. A customer who does not get a password reset calls; a customer who does not get your newsletter notices nothing.

How to do it

A separate sending channel, and preferably a separate sending subdomain. Reputation is then built independently.

What else it changes

You can monitor transactional mail more strictly and set different alerts. A delay on confirmations is an incident; a delay on a campaign is not.