You can resend an existing campaign when it was originally sent before 45 days. resend to specific contacts.

To resend to newer contacts to the same list:

1. Click on the Emails tab.
2. Click on the Email Reports link from the Reports menu.
3. Click on the email campaign to from the list of campaigns to resend
4. Click on the “Resend Email” button from the Report Summary Screen
5. You will now have an option to resend to newer contacts or to new email addresses
6. Select the “New Contacts since Email was last sent“
7. You can now either select “Send Immediately” OR reschedule the email campaign. In case now new contacts have been added, the campaign will not be sent.