Undelivered Mail Returned to Sender

Starting some time yesterday morning, emails that are forwarded by rules from iCloud mail have been returned to sender. I get an email from iCloud with subject "Undelivered Mail Returned to Sender" with a lot of unreadable text starting with

<REDACTED@comcast.net>: host mx1.mxge.comcast.net[96.102.18.146] said: 530
   5.1.0 <ALSO-REDACTED@icloud.com> sender rejected (in reply to MAIL FROM
   command)

This just started yesterday morning. Forwarding was successful until then. 

It seems to have been corrected a few minutes ago.

What happened, and will it happen again?