You have at least 2 choices for choosing the key used by ssh. Taking
email@example.com as the example here.
Method one, specify the key in command line with the
-i option of ssh. Check more in the manual.
The command line to be used is:
ssh -i ~/.ssh/key1 firstname.lastname@example.org
You can add to your
~/.ssh/config lines as follows.
Host example.com IdentityFile ~/.ssh/key1
This tells ssh that for host
example.com, use the key file in