You can send emails simultaneously to multiple contacts using Stager. These email addresses are separated by commas. Outlook on Windows uses a semicolon as a separator by default, so you get an error message when you use Outlook. Read here how to set up Outlook, so that you can also send emails using it. Scroll down for more recent versions of Outlook.
Outlook 2007
Open Outlook and click on 'Tools' and on 'Options'.
In the 'Options' menu, click on 'E-mail Options' under E-mail.
Then click on the 'Advanced Email Options' button.
In 'Advanced E-mail Options' you see the option 'Allow comma as address separator'. Tick it and click on 'OK'.
Now Outlook can read both commas and semicolons as an email separator.
Outlook 2010/2013/2016/365
Click on 'File' and 'Options'
Click on 'Mail' on the left side and scroll down until you reach 'Send messages'.
Here you find the option 'Commas can be used to separate multiple message recipients'. Tick it and press 'OK'.
Now Outlook can read both commas and semicolons as an email separator.