From HollowWiki

H-Mail stands for HollowMail. This works in a very similar way to your standard e-mail. You can use it to send private messages to players and admin. You can send the message to multiple players by placing a comma between their names. Spamming players and admin with unwanted or repeated H-Mails is an offense.

H-Mail is a very multifaceted part of Hollow. The messages can be used as a means of OOC communication to set up RP times or do anything else you would do in an e-mail. This system can also be used if you wish to report a violation of the rules to an admin, or if you'd like to apply for a second character (alt), a custom item, or to have a room built in the world called Hollow. You can also use h-mail to save copies of special RPs you've participated in or witnessed and send them to other people.

Aside from typical e-mail applications, the H-mail system can be used for a variety of IC functions. Using a subject such as "{A letter is pinned to the front door of your house.}" allows the possibilities of sending and receiving mail ICly, which makes keeping in contact with other characters far easier. Currently, there is no framework set up to attach items to h-mail, but other than this small limitation, the only restrictions of RP through h-mail are those of your imagination.

See also

Command List