Antivirus software will occasionally disconnect an email client (your computer) from the correct path to the mail servers, especially after an antivirus update. First, close the email client. Open the antivirus options/settings. Turn off all options that check sending or receiving mail for viruses, apply and close. Some anti-virus clients change the mail server path in order to check for viruses. Open the email client and verify that the email server settings are correct:
Usually, this setting is under tools/email accounts/properties. Default settings for POP/IMAP are mail.yourDomainName.com and for SMTP is mail.yourDomainName.com or your Internet Service Providers SMTP server, apply and close the window.
At this point, test sending and receiving, if the anti-virus software was the cause of the problem, you should be able to send an email to yourself. Finally, open the antivirus options/settings and turn back on the options to check sending and receiving email for viruses.