Question:
We host our website with 3Essentials, but our mail services on our own network on our own mail server.  Can 3Essentials be configured as a "fallback/failover" or secondary email service in the event our mail server is down?

Answer:
Yes, with some limitations.  To implement, you would update your DNS MX records in this fashion:
- an MX record with high priority pointing to your mail server
- an MX record with lower priority pointing to our mail server

For example:
yourdomain.com   MX(10)   yourmailserver.yourdomain.com
yourdomain.com   MX(50)   ourmailgateway.3essentials.com

*Note that a LOWER MX number indicates a higher priority.

Also, note that our "gateway" server may be different depending on which server you are hosted on, so please submit a support request asking for the name of the 3Essentials mail gateway your MX should be pointed to, and reference this KB article.

So, after you make the necessary changes to your MX record, this means that if your mail server is up, mail will be routed to your mail server... if it's not available, mail would be routed to us.  There is one more step you need to take... you actually have to create a mail user on our servers for EACH mail user you want to receive mail for.  So, if in your company and on your mail server you have accounts for:

mary@yourdomain.com
john@yourdomain.com
bill@yourdomain.com

Then those mail accounts must ALSO be created on our mail server.  For instructions on how to create a mail user on our servers, please refer to the following article:
http://knowledge.3essentials.com/index.asp?menu=a1_2&go=article&id=78

Once you've created the mail users on our servers, we will accept mail for those users if mail is routed to us for those users (presumably only if your primary mail server is down).  This means our server will accept those mail messages and HOLD them until each user retrieves them - we cannot/do not provide STORE AND FORWARD functions... i.e., to forward the mail BACK to your mail server once your mail server is back online.   This means you will have 2 options for accessing the mail messages on our servers if mail has been routed to us:

1. on your primary mail server, if it supports POP RETRIEVAL, have your mail administrator configure each mail user's mailbox to POP RETRIEVE mail messages from our server.  That way, once your mail server is back up, it would POLL our server and pull any mail messages over to your mail boxes. 
2. while your mail server is down, your mail users can access their mail accounts on our servers via webmail at http://webmail.yourdomain.com.  Be sure when they login that they specify the domain in the login/username... i.e., john@yourdomain.com, not just "john".