Exim: The Mail Transfer Agent

Chapter 12. Delivery Errors and Retrying

This chapter is all about temporary delivery errors, and how Exim deals with them. In an ideal world, every message would either be delivered at the first attempt, or be bounced, and temporary errors would not arise. In the real world, this does not happen; hosts are down from time to time, or are not responding, and network connections fail. An MTA has to be prepared to hold on to messages for some time, while trying every now and again to deliver them. Some rules are needed for deciding how often the retrying is to occur, and when to give up because the retrying has been going on for too long.

A related topic is how to handle messages destined for hosts that are connected to the Internet only intermittently (for example, by dial-up lines). In this case, incoming messages have to be kept on some server host because they cannot be delivered immediately. Exim was not designed for this, and is not ideal for it, but because it is being used in such circumstances, the final section of this chapter discusses how it can best be configured.

Retrying After Errors

Delivering a message costs resources, so it is a good idea not to retry unreasonably often. Trying to deliver a failing message every minute for several days, for example, is not sensible. Even trying as often as every 15 minutes is wasteful over a long period. Furthermore, if one message has just suffered a temporary connection failure, immediately trying to deliver another message ...