You can set up IMail Server to act as a backup spooler for a customer's mail server. If the customer's computer is down, the mail for his domain will collect on your IMail Server until his is back up. The customer's mail server must have a static, unchanging IP address.
To configure this, have the customer set his computer up to log into his ISP at intervals where he can catch the queue processing interval (Times Before Return to Sender setting on the SMTP Settings page) of your server. For example, if your retry timer is set for 30 minutes, have him connect to his ISP once every 20 minutes or so. He has to be online and ready to receive when your timer cycles.
Alternatively, he could Telnet to port 25 (the SMTP port) of your computer and issue the ETRN command with this format:
etrn his_domain.com
This will dump the queued mail to his computer.
In DNS, your server will be the secondary MX for his domain (lower priority), whereas his server will be the primary MX.
You must also make an entry in your hosts file (\WINDOWS\system32\drivers\etc\hosts) which associates his IP address with his domain name. For example:
his.i.p.address his.domainname
This way, IMail Server attempts to deliver mail it receives for that domain to his computer, bypassing the MX records in DNS, which points to itself and can create a mail loop.
If you are using IMail's SMTP security to prevent spammers from using your computer as a spam relay , add his server's IP address to the Access Control page.
For example, if the remote host's DNS is set up to receive mail for a primary domain, such as mail.widgets.com, and points to your IMail Server, mail.domain.com as a backup server, then the MX record for the remote host's DNS will look like this:
MX
10 mail.widgets.com
20 mail.domain.com
When mail.widgets.com is down, mail is sent to your computer mail.domain.com. To relay mail for the mail.widgets.com domain, you must specify its host name and IP address in the hosts file on your IMail Server host.