For tickets to be created in request tracker queue by email, aliases for the different queues should be created in the /etc/aliases file. Pipes in the aliases file should be enabled in the MTA, they are by default in Postfix. Then the alias pipes to the rt-mailgate program with the queue name as an argument like the following:
rt: "|/usr/bin/rt-mailgate --queue General --action correspond --url http://rt.prgmr.com/rt" rt-comment: "|/usr/bin/rt-mailgate --queue General --action comment --url http://rt.prgmr.com/rt"
The alias is usually something related to the queue name but the queue name can have spaces in it so it might be different. If the queue name has spaces it should be in escaped quotes like \"A queue name with spaces\" so that the quotes don't match the quotes around the command to run in the alias file. Each queue has two aliases and the -comment alias is for making a Comment on the ticket which in Request Tracker the requester of the ticket might not be allowed to see.
For more information on setting up reuest tracker, the following articles are helpful: