We have a perl program that generates gpg encrypted files for multiple folks.
Today, we added dan@example.com. When I run the encryption script, it happily encrypts the file for dan@example.com using jordan@example.com. This is presumably because jordan@example.com was in my keyring first.
I've tested from the command line, and if I use gpg -r dan@example.com --encrypt foo.txt, the friendly client chooses to use the public key for jordan@example.com, instead of the more explicit dan@example.com.
Of course, if I use key ids it works, but that's not really what we've got here. Is there a way to tell gnupg to use the more explicit email address without resorting to key ids?
This is documented in the manual page that comes with gpg. Section "HOW TO SPECIFY A USER ID" you find the option "By exact match on an email address.". So the answer to your question is to use angle braces such as <dan@example.com>.