Subject: some hints
The 15 K Limit is most likely simply the limit of the field in the forn itself, nothing else. In a rough calculation 15K would equal 15000 characters. Divided by 350 addresses, this would mean that one address could be about 42 characters long, each. I did never testing in detail, but I would guess that there is some overhead to deduct for storring the multi values propperly in fields. => Lets say it is fairly save to say for 350 recepients, that one of them could be up to something like 35 or maybe even 40 Characters long .
Now have a look at the following sample address - which is on the one side probably longer than everage, but on the other side not even close to the longest of the possible formats:
Firstname.Lastname@ThisSpecialCompany.com
123456789 123456789 123456789 123456789
I have not tested the other part of the answer, either, but I guess, the 5 MB means groups only for the case where the mail ends up in mail.box still with not expanded groups.
(when the groups are expanded could be checked after a tell router quit - don’t forget to load the router at the end again. If the mail is taken back via cut, copy and paste to a mail template, it should open up again, nicely. If the groups expand for you on the server, but for the user on the client, the user might have done a copy of the group to their local adressbook or something. ).
In theory a group has a length, and again another 15K Limit in the Members field, and groups could be nested - I think up to 6 levels deap (not sure if that limit still exist). I did not do the calculation, but it would end up with way more than 5MB of entries (group names can be short), and therefore this limit is most likely comming from somewhere else. Getting close to the problem from another angle: If 15 K is 350 People, 15 MB would be 350 000 people (or 5 MB about 100 000). This is more mails than a lot of companies with 5000 notes seats would send on the avarage day. And this one single mail would moste likely keap the server (or its smart host) busy for hours - I hope it is getting send with low priorty to be proccessed after business hours?. If this would be a big mail in a place with slow infrastructure and a backup system taking down the server every night, most likely this server would never ever complete sending this mail!
I hate spamers, and hope nobody will go for this, but a simple client side Lotus Script agent sending the mails one by one could definitely go way behind this.
Back to your users: I would simply tell them to send to about 200 people or less at a time, send them low priority to allow sending them at night (send the same mail to the second part of folks afterwards) for system limitations or that you would need money for a project …