Testing a campaign before you send
A mistake in an email cannot be corrected afterwards. Ten minutes of testing saves you an apology message.
Test the links
All of them, including those on images and in the footer. A dead link in the main button makes the whole campaign pointless.
Test the personalisation
Send yourself a version with a missing first name. "Hello ," is the most visible and most common defect.
Test across clients
At minimum a webmail, a phone and Outlook. Those are the three environments where the differences show.
Reread the subject and the sender
Those are the only two things everybody sees. A typo in the body is annoying; a typo in the subject is visible to your entire list.
Have somebody else read it
You stop seeing your own mistakes after the third read. A fresh pair of eyes finds in two minutes what you have been hunting for an hour.