What is the maximum number of recipients per email message?

Email
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.