This may help

Details

In order for e-mails to be scanned by the Mail Shield, it is necessary that encryption is turned off in the mail client software. If encryption is required, the appropriate e-mail accounts need to be configured in the Mail Shield to establish a secure connection with the mail server. This will enable all incoming and outgoing e-mails to be scanned by the Mail Shield. All other e-mail accounts using unencrypted connections should be listed as unencrypted in the SSL Accounts settings of the Mail Shield (i.e. SSL/TLS encryption set to ‘None’, ports for SMTP to ‘25’, for POP3 to ‘110’ or for IMAP to ‘143’, by default).

There is a lot more on the links below

[url=https://support.avast.com/index.php?_m=knowledgebase&_a=viewarticle&kbarticleid=458&nav=0,2,64]

https://support.avast.com/index.php?_m=knowledgebase&_a=viewarticle&kbarticleid=458&nav=0,2,64[/url]

https://support.avast.com/index.php?_m=knowledgebase&_a=view&parentcategoryid=64&nav=0%2C2