I looked at this recently in the german speaking section E-Mail drucken von Fairphone /e/ - #4 by tcecyk
A workaround is to install org.billthefarmer.print app found in fdroid / Apps to have a “Share” entry (as proposed in the k9mail forum).
From there you can reach any IPP enabled network printer you’d also have via a real printer function.
But it has a major disadvantage: k9mail doesn’t yet share the essential mail headers to / from / subject and date - so at least #1423 needs to be fixed and the /e/ fork rebase to be a true solution