I recently deployed DKIM on a number of my domains. For those readers of my blog that are unfamiliar with DKIM (Hi Mom and Dad, I love you.), DKIM is just a fancy way to stamp emails with a special digital signature. DKIM makes it so other mail programs can inspect the message and determine if it really is from me.
I also manage a number of email lists, and I subscribe to a number of other lists. Email lists have a habit of appending trailers with helpful instructions for managing subscriptions, and adding prefixes to the subject. This altering of the message as it passes through the mailing list invalidates the DKIM signature.
Today I tested a “fix” for one of my Ezmlm mailing lists with these commands:
cd path/to/ezmlm/list; rm prefix text/trailer addtrailer
Then I sent a test email to the list, and voila, the message passes DKIM validation.