Could you please let us know whether do we send/receive errors ?
What is the email client that you use for mails ? (Eudora/outlook/thunderbird) ?
Open security center and let me know whether it states Am I protected YES or Your computer is secure ?
Open security center and click Internet & network
Click configure (to the right side)
Click on Program permission from the left pane
And check whether Your email client is been given full access under the programs permission list .
If yes,, turn off the McAfee firewall and check whether you are able to send emails .