Set all mailboxes to read only

I need to set all of my Exchange 2010 sp3 mailboxes to read only. I found an article by Praveen that describes the process, but it appears to be doing so on a per user basis. I'm not familiar enough with powershell to know how to modify the commands to select all mailboxes.

Can someone please tell me how to set all mailboxes to ready only with powershell?

Thanks!