We don't have synonyms for postmarked yet — it may be a rare, specialized, newly coined, or misspelled word. The closest thesaurus entries we have are pockmarked, postmark and postmarks.
(n) :
A marking made by a postal service on a delivery, usually indicating the place where and the date and time when the item was received or processed for the first time, mostly serving to cancel a postage stamp.
(n) :
A marking made by a postal service on a delivery, usually indicating the place where and the date and time when the item was received or processed for the first time, mostly serving to cancel a postage stamp.