Set up customer's e-mail addresses in Plesk. Worked fine in testing, all goes well for about a week.

Then their e-mail stops delivering. Stuff arrives, but outgoing messages either bounce or fail silently altogether. I contact 1&1 support, and they help set up SPF and DMARC on the domain, and then we wait and see once the DNS changes propagate.

Well, something about these changes caused my business e-mail (on a separate server) to exhibit the same problem now, when it had been working for 3 years without issue prior to that.

Check back with 1&1 2 days later to see why the first one isn't working; we verified all of the records across everything, tweaked a couple other things (like setting the full hostname in Plesk to mail.servername.com), and waited 2 more days.

Still having the same problem on both accounts. did a bit of looking up the issue for Plesk and found that in order for SPF/DMARC to work, they have to be activated on the Plesk-wide mail settings, and then again individually at the domain level.

Made these changes on my business e-mail's server and domain and it fixed the problem!

Made the same changes on the server with the customer's domain and...still seeing the same issue.

Have checked all settings between them and they're identical. All the appropriate DNS records are in place. I'm kind of at a loss for waht else to check at this point.