trouble sending email to large address book group

I am unable to send a group email. I've used this group before successfully.

When I type the group name into the BCC bar, it responds a little differently than other groups; it appears in boldface, and does not expand into visible addresses. I've tested with other groups (none as large) and have no problem.

I'm using OS 10.6.8. My email is version 4.6 (1085). Should I just try making it into two smaller groups? Is there an upgrade I'm missing?

Thanks.