To add on my previous message, also run the linter like below and try
to fix the warnings:

$ ./pre-inst-env guix lint libmilter sendmail
gnu/packages/mail.scm:2871:2: libmilter@1.0.1: 'm4' should probably be
a native input
gnu/packages/mail.scm:2871:2: libmilter@1.0.1: file names of patches
should start with the package name
gnu/packages/mail.scm:2891:14: libmilter@1.0.1: no period allowed at
the end of the synopsis
gnu/packages/mail.scm:2871:2: libmilter@1.0.1: permanent redirect from 
http://sendmail.org to 
https://www.proofpoint.com/us/products/email-protection/open-source-email-solution
gnu/packages/mail.scm:2871:2: libmilter@1.0.1: source not archived on
Software Heritage
gnu/packages/mail.scm:2858:5: sendmail@8.16.1: 'm4' should probably be
a native input
gnu/packages/mail.scm:2860:15: sendmail@8.16.1: permanent redirect from
http://sendmail.org to 
https://www.proofpoint.com/us/products/email-protection/open-source-email-solution
gnu/packages/mail.scm:2772:5: sendmail@8.16.1: source not archived on
Software Heritage

Thank you!