Pretty sure they will time out and disappear on their own.
Best bet for an email relay server when you run into something like this is to get a really cheap PC (I have used even a PENTIUM II-450 with 512MB RAM) and install MSL with no blades, do not register it with AMC, then use it as just an email relay server, works super slick. Just set it up to accept local only mail injections, point the 5000 (3300 or any other SMTP sending device) to it as it's mail server and let it handle all the relaying. This is also very handy sometimes as a DHCP server for the voice VLAN since the 5000 does not have an embedded DHCP server. Lately I have been picking up Pentium 4 PC's with 1GB RAM and an 40GB hard drive at the local state surplus place for $40, works great.