Did you change all the settings under the 'Advanced' tab ? If you could receive the emails you should be able to send emails too. I suggest you to empty your 'Outbox' folder, and then try to send again.
Also, If you have a firewall installed, please make sure that its settings allow IncrediMail to access the Internet, by making sure that the following processes are in the list of the firewalls approved programs:
If you are still unable to use IncrediMail, please try disabling your firewall completely.
IncrediMail Customer Support