[exim] combining multiple incoming messages to same local do…

I use exim 4.76 and mysql to provide a maillist for an organization
where individual members are assigned to one or more groups. Staff
members may then send a message to one or more groups with the group(s)
being listed in the To field, all at the same domain. The previous
ISP's SMTP server for the organization delivered a single message to the
exim server with possible multiple groups as recipients, and exim is
smart enough not to send duplicate messages to an individual when he may
be in more than one of the groups. The organization has switched to a
new SMTP service, and now a single message to multiple groups, all at
the same domain, is being split into separate messages (one for each
group) by the SMTP host. When these separate messages arrive at the
exim server, it is no longer able to suppress multiple messages to the
same individual. For example, if John Doe is a member of group1 and
group3, and a message is sent to group1@???, group2@??? and
group3@???, John Doe receives the message twice.

Does anyone know a way for exim to combine such messages and process
them as a single arriving message addressed to multiple groups? Factors
for combining would include a short time window (ie, the split messages
arrive within seconds of each other), the same sender, and the same subject.