What is the Plural of mail?

The plural of the noun mail is mail. The verb is mails or mailed . He/she mails the correspondence for her office. Rarely used I think.

mail is both singular and plural

that is so interesting! you are right wiki says it is mails.
but its 3rd person singular whatever that means~!

1. Third-person singular simple present indicative form of mail.

http://en.wiktionary.org/wiki/mails

No, it is mail.