The Latin word "corpus" (which means "body" in English) is frequently used to designate a collection of messages, either ham or spam.
The plural of "corpus" is "corpora".
Arguably, English will allow the use of "corpuses", but it looks and sounds a little odd.
The plural of "corpus" is NOT "corpi" or "corpii" or anything like that.
(For the linguists, "corpus" is a 3rd declension neuter noun.)
On a related point, the English plural of "virus" is "viruses" .