Having a look at an issue I noticed problem with SMTP: Bro assumes that e.g. the To-field contains a comma-separated list of mail-addresses. According to RFC 5322 there is also the possibility to use groups (see below).
Regarding groups I am not sure whether they can be nested. If I am not mistaken, the grammar in the RFC would allow nested groups. But in my understanding this is not desired for the Destination Address Fields:
the field name, which is either "To", "Cc", or "Bcc", followed by a comma-separated list of one or more addresses (either mailbox or group syntax)
That leads to two questions:
Would it be sufficient for Bro to log just the addresses (usually whats inside < and >) without description (quoted with " )?
Should Bro support nested group-syntax?
I think option 1 (just log the plain addresses) should be sufficient, because if someone is interested in more details, one could have a look at the raw headers.