Are you collecting you email from a POP or IMAP box or are you running an incoming mail server with an MX record for the domain pointed to your ip?
For outgoing mail and collecting POP/IMAP mail you might achieve what you want with a load balancing system that spreads traffic between the two connetions. The load balancing software Gnu zebra
has been mentioned in these forums before. I havn't used it myself.
If you are running you own incoming mail server then you need two MX records for the domain. If you felt ambitious you could have the DNS record updated to set which MX is highest priority depening on which internet connection is up, don't know how to set that up though.