There is an issue that has been closed out on Github without a resolution which details a problem with emails sent via posts. I’m also running into the problem with my emails. The problem is that the site logo does not have an alt tag. According to https://www.mail-tester.com/, the recommended email quality checker, this negates 0.5 of 10 possible points from the email score. This means by default, all emails are performing 5% worse than they should be.
There are options for setting an alt for images you insert yourself, but not for the auto-inserted logo. I think a reasonable solution would be to either default the logo alt text to the site title, or allow users to set it themselves in the email customization tab.
I am hitting this issue from the Ghost Pro account, not a self hosted instance. The Github link below has all of the details in place for replicating the issue.