xatra -xa'a- gismu

x1 is a letter/missive/[note] to intended audience x2 from author/originator x3 with content x4.

(cf. notci, which has places in a different order; the emphasis in xatra is on the communication between author and recipient, and not the content, which in a letter may not easily be categorized to a 'subject'; ciska, mrilu, papri)

In notes:

ciska
x1 inscribes/writes x2 on display/storage medium x3 with writing implement x4; x1 is a scribe.
mrilu
x1 mails/posts [transfer via intermediary service] x2 to recipient address x3 from mailbox/post office/sender address x4 by carrier/network/system x5.
notci
x1 is a message/notice/memorandum about subject x2 from author x3 to intended audience x4.
papri
x1 is a [physical] page/leaf of book/document/bound mass of pages x2.
fairbei
b1 distributes b2 to b3 from b4 over path b5. b1 is a distributor.
venxa'a
x1 is a mailer about v2 from x3=v1 for audience x2=v3.
venxatrycu'u
c1 the direct mail business/activity/function for goods/products/services/activities v2 from/by x3=v1 to audience x2=v3.