Hello David,
I can understand your concern. Please disable AVG temporarily http://support.avg.com/SupportArticleView?urlname=How-to-use-Passive-Mode and check if you are able to send emails. If you face any difficulties, feel free to contact us.
Why is AVG BLOCKING out going plain text e-mail?
Win 7 pro
ThunderBird e-mail client
AVG 17.5.3022 (Aug 15,2017)
We are suggesting to disable "Outgoing scan" temporarily to check the cause of this issue.
We will re-enable this once we identity the issue.
What if I want my outbound mail to be scanned AND sent?