Following are the important parts of an actual (very small) delta piece:
Subject: ctm-mail src-cur.0003.gz 1/4
CTM_MAIL BEGIN src-cur.0003.gz 1 4
CTM_MAIL END 61065
The subject of the message always begins with "ctm-mail" followed by the name of the delta, which piece this is, and how many total pieces there are. The data are bracketed by "CTM_MAIL BEGIN" and "CTM_MAIL END" lines, duplicating the information in the subject line, plus a simple checksum.
If the delta exceeds maxctmsize, then a message like this will be received instead:
Subject: ctm-notice src-cur.0999.gz
src-cur.0999.gz is 792843 bytes. The limit is 300000 bytes.
You can retrieve this delta via ftp.
You are then on your own!
If deltas are to be applied then ctm(1) and gunzip(1) must be in your PATH.