The steps to reproduce the bug are:

1. Look at the Partner list e.g. go to Sales > Address Book > Customers.
2. Select *multiple* Partners by ticking the checkbox next to them.
3. Click the "Mass Mailing" link in the right-hand sidebar.

This time you will see that the From and To fields are blank and mandatory, which doesn't make any sense. Also, you cannot click the message template button as it excepts something to be entered in the From, To and Subject fields.

This wizard needs to be redesigned to allow more than one Partner to be emailed using a single message or template.