Content-Type: text/shitpost

Subject: What comes next in this sequence?
Path: you!your-host!walldrug!epicac!ihnp4!hal9000!plovergw!shitpost!mjd
Date: 2019-03-30T20:36:39
Newsgroup: misc.abcd
Message-ID: <fc6a984ff1facaa5@shitpost.plover.com>
Content-Type: text/shitpost

Anal
Banal
Canal
????

I mean, obviously “danal”, but what does it mean?