Hello Louise,
Thank you for reaching AVG support channel. We are sorry for the inconvenience caused. We understand that you are unable to send and receive emails when AVG Internet Security is ON. We really appreciate your efforts. We will check and help you to resolve it.
1. Could you please confirm whether you have completed all the updates for Windows?
2. Also, please check whether AVG Internet Security program is upto date. If not, please follow the below article to update it :
Updating AVG AntiVirus
Once you complete all the updates, please restart the device and check whether you can send and receive emails when AVG Internet Security is active. If you still have issues, please do let us know. We will help you to resolving it through advanced remote support. We look forward for your response.
A week ago my Outlook 365 stopped sending and receiving emails. When I disabled the Email Security in AVG, I could receive emails, but still can't send them. I followed the information here: How to troubleshoot email client warnings about invalid server certificates | AVG but on answer 8 near the end of the page it says "Click More choices, then select the certificate you want to use." My version of outlook doesn't offer choices, it just tells me I have no valid certificates to choose from (probably because the earlier instruction exported the certificate that had been installed). I've downloaded the file MailShield.der to my desktop and opened it, allowing the "wizard" to install the certificate, all to no avail. If anyone has any ideas, I'd be so appreciative. Technology is NOT my first language 
and I feel like superwoman for having gotten this far. I just need a leg-up to get through to the end of this process.
P.S. This sort of drama does not encourage me to pay for the full version of AVG. Just saying…
Hello Louise,
Thank you for reaching AVG support channel. We are sorry for the inconvenience caused. We understand that you are unable to send and receive emails when AVG Internet Security is ON. We really appreciate your efforts. We will check and help you to resolve it.
1. Could you please confirm whether you have completed all the updates for Windows?
2. Also, please check whether AVG Internet Security program is upto date. If not, please follow the below article to update it :
Updating AVG AntiVirus
Once you complete all the updates, please restart the device and check whether you can send and receive emails when AVG Internet Security is active. If you still have issues, please do let us know. We will help you to resolving it through advanced remote support. We look forward for your response.