Accept e-mail replies from a secondary e-mail address

Problem:
Many people use two different e-mail accounts when replying to a ticket. One on the computer and one on the mobile phone but because RSTickets only accepts replies from one e-mail address, some replies get rejected.

Request:
If there were an option to add a secondary e-mail address (per user) to accept replies from, the ticket system could accept replies from both the original and the secondary e-mail. The message would of course still need the correct ticket code in the subject.