My email program did not complain about the number of names, at one point it was over 500 before the wards where split. The problem I faced was the mail server that I was using though I was trying to spam people since I had so many people listed in the bcc (blind carbon copy) section, so no-one could see who else I was emailing it to. To get around the problem I started off as normal, yet I selected and copied all of the email addresses from bcc and pasted it into a blank notebook page. I then could cut, copy, and paste as I needed into the bcc address line. So, these are the steps I would take.
1) Compose regular broadcast message, selecting who needs to see it (in this case, all adults), and proceeding in a normal fashion.
2) When my email program would open I would cut the address from the BCC address line
3) Open Notepad, and paste the addresses.
4) Through trial and error I learned that I could select about three lines of address, cut them, and paste them back into the BCC address line of my email program
5) Hit send, and see if my mail server generated an error about too many people being addressed, or sending to many emails to quickly
6) Adjust the cut and paste from Notepad to my email program as needed.
7) Open the sent emails folder, double click on the email I just sent so I can edit it.
8) Once the sent email is opened I can select all of the addresses in the bcc address line and replace (paste over) them with the next group of email addresses.
9) Repeat steps 4, 5, 7, and 8 as needed
And it may just be your email program, and not the postal server, that is limiting you in being able to send out a batch email. The same steps should work in either case.