Postmarking

from The Collaborative International Dictionary of English v.0.48

Postmark \Post"mark`\, v. t. [imp. & p. p. {Postmarked}; p. pr.
   & vb. n. {Postmarking}.]
   To mark with a post-office stamp; as, to postmark a letter or
   parcel.
   [1913 Webster]