I had the same problem when I still had Norton antivirus (before getting Kaspersky). When trying to send e-mails, I get a message stating that my SSTP server doesn’t support encrypted messages.
With Norton I was able to turn off the outgoing mail scanning feature, which fixed the problem. Unfortunately, there is no menu item in Avast for e-mail, so I can’t do anything about it (or so it seems).
Ensure that you have uninstalled previous AVs and all remnants of the other AVs .
A link worth looking at, which is a program removal tool that can remove the remnants of a number of different Norton Programs: Removing your Norton program using SymNRT
Is that MS Outlook (not express) ?
If so make sure the avast plugin is not disabled in MS Outlook:
Outlook 2007 > Tools > Trust center > Add-inns > Manage Disabled items.
Outlook 2003 > Help > About > Disabled items
Outlook 2000 > Tools menu > Options > Other > Advanced Options > Add-In Manager.
I do not wish to disparage the post of Thunderbird above which is, I believe, intended to answer the question put by the original poster.
It is nevertheless not applicable if the original poster follows the recommendation of the avast team to use the far superior Outlook/Exchange provider with Outlook. The use of this provider makes the Internet Mail provider null and void for Outlook.
Thank you so much, guys! I followed David’s advice for Outlook 2003, and found that avast was disabled. I enabled it again, and now it is working fine! I am glad I posted here.