If you want to send a mass mail to all clients, only clients on a specific domain, or just a comprised list of accounts, you can do this by setting up a mailing list. This mailing list can be told to mail all accounts listed on the server, with any message you desire.
To create this list go under a specific domain, or just the primary domain on the server if you are mailing to everyone on the box. When there hit [Control-M], this will open a blank mailing list window. You then need to name the mailing list, for example (allusers@domain.com) could be a name of the list. Now just continue to fill in the fields as specified, the owner is going to be whomever will manage and control the list. When you get to the [Source] field you can then choose from a few options, in the middle of the drop down menu you will see ( All System Users ), this will email everyone on the server. The other options depend on what you are looking to do with the mailing list, as you will see (All users in current domain), choose this if you want to only send the domain you are creating the mailing list under. If single users are chosen then whom ever is a member will show on the members tab.
With just the information you already have provided, the mailing list will work. The other tabs let you tweak the maintenance of the mailing list and also define other options, such as who the mail be returned to if someone reply's to the mail. You will see these fields under the [Message] tab, The first field lets you designate who the message will appear from, if you choose (Set to sender) then this will show who exactly sent the mail. So if you want the mail to not show coming from your personnel account you would not choose this option. The next option (Set to Value) will let you specify an address to show who the mail came from, for example, (support@domain.com) would show as the sender if you set the value to this address.
The next field is for the reply mail, this is the same as the [From To] option in regards to the options in the drop down. Once again you can either set this to the sender of the message or define an address to accept the reply messages. The one difference being this, you can set a totally different address for the reply mails, then you defined as the sending address. So if you want the mail to show as coming from support but want have any reply sent to your inbox then you would [Set Value] as your address.
The next option is if you want to make the list password protected. There are two choices, the first [Server Moderated]. If you choose this option you will need to then assign a password for the list. This will now take security of the list even higher, now even members would need to add the password to the subject of the message they are sending out. So if the pass was "mail" the subject would need to look as follows:
The difference between [Server Moderated], and [Client Moderated] is under the server moderation, any mail that is sent to list with either an incorrect password, or no password at all is sent directly to the owner of the list for conformation. This would go to their inbox and then they could choose whether the mail is legitimate or not, if they think it is, all they need to do is reply and the mail will go through to the list.
The last tab is [Rules], here you can either black or white list mail, addresses, etc. This will work the same way as all other B/W lists on the server.