The maximum number of recipients per message for mail sent via webmail or through the outgoing SMTP servers is 50
So this limit will apply not only to webmail, but also to your local
desktop email program and web application that uses
mail.yourdomain.com as the outgoing mail server.
Messages with more than the maximum number of recipients will not be
sent. The recipient limit is in place to help prevent spam originating
from our servers.
To work around the 50 recipient limit when sending mail from your local
desktop email program, we suggest using your ISPs outgoing mail server.
Some of the more common ISP SMTP servers are listed in this Knowledge Base article
. If your ISP is not listed, contact them for the proper SMTP setting.