Most likely the issue with IncrediMail is caused by the improper account configuration on your PC or because the software is not allowed to communicate with the Internet servers because of the built-in firewall. I recommend checking the account settings and eventually re-add the account, but beware that removing an account will also delete other emails, therefore create a backup.
Additionally, switch to Thunderbird as it's more reliable than IncrediMail and provides additional info.