How should Exim be configured when it is acting as a temporary storage system for a domain on a dial-up host?

Exim isn't really designed for this, but... The lowest-numbered MX record for the domain should be pointing to the dial-up host. A higher numbered MX record (lower priority) should point to the Exim server that is acting as a temporary storage system. You should set a large retry time for the domain, so that Exim doesn't keep trying to deliver when the host is offline. When the host comes online, the waiting messages have to be kicked somehow. This can be done by calling Exim with the -R option, or via the SMTP ETRN command. This works provided the number of messages is low. If you are handling lots of mail, keeping messages waiting for their host to connect and those that are having delivery problems to remote hosts all in the same queue doesn't work so well. It is better in this case to get Exim to deliver the mail for the dial-in hosts into some local files which then get transmitted by other software when the host connects. One tool for doing this can be found at http://cr.yp.to/serialmail.html. For further discussion, see section entitled Intermittently connected hosts in the manual, and also the section in the Exim book with the same name.