Using semicolons as address separator gives uninformative error message
|Reported by:||greig||Owned by:|
If you type two or more addresses in the To: (or any recipient field), and use the incorrect but popular (thanks to Outlook) semicolon as an address separator instead of the correct comma, the error message is simply "Failed to send message". This will confuse people used to Outlook.
Suggest changing the error to something more informative.