The problem has resolved itself somehow. But in case it helps others I'll explain what happened.
I've set up a mailmap for my domain in ZoneEdit (have had it for years). The other day, whenever I tried to send mail to an email address on my domain, the message would be undeliverable. The email address where things were mapped to was a comcast.net address. I thought that the problem was with Comcast, that they somehow were blocking email from forwarded addresses. I talked with them, but they didn't want to deal with me because the original address being sent to was not a comcast.net address. I then realized I should try to set up a mailmap that would forward to a gmail.com address to see if it really was Comcast. But email through that didn't work either.
I'm not sure what changed, but several hours later the mailmap here in ZoneEdit started working again.