Microsoft outlook occasionally has an issue with receiving multiple email recipients from external programs like System Five. Even though the format is correct in the address line ( email@example.com; firstname.lastname@example.org ), you may still receive the following error:
The workaround is fairly simple. In System Five, instead of using a semicolon between email addresses, use a comma. Then in outlook, click on File, then Options. This will open the Outlook Options window. Now click on Mail in the left hand window, and then put a check mark in the check box next to “Commas can be used to separate multiple message recipients”.
Restart Outlook and you shouldn't have any further issues as long as you use a comma to separate multiple email addresses within the email field in System Five.