We don't have synonyms for mailgrams yet — it may be a rare, specialized, newly coined, or misspelled word. The closest thesaurus entries we have are mailgram and milgram.
(n) :
(telegraphy) A telegraphic message transmitted electronically from the sender to a post office and then printed and delivered to the recipient via physical post.
(n) :
(telegraphy) A telegraphic message transmitted electronically from the sender to a post office and then printed and delivered to the recipient via physical post.