Puzzling challenge: my own certificate not trusted?

[Johnsond72]

For the first time in using my email account on a Mac over ten years, yesterday got a message after sending an email about my certificate not being secure. I verified the email was actually sent and received. Poking around the messages, the trail seemed to lead to a TLS certificate. When I clicked a box saying "Always trust" the problem message vanished. However, now all my outgoing emails from this account have a new line just below the To line in the header. It says: "Security: [a check mark in a small black badge] Signed: (Douglas M. Johnson)" 

Since my only recent change has been to begin using Malwarebytes Privacy with the VPN turned On, I'm wondering if that has caused this change. I also installed the recent macOS v11.3.1 so I cannot be confident which is causing the changed behavior. 

Am I at risk for telling my own computer to always trust my own certificate? Seems pointless although I'm wondering if this issue is related to the VPN encrypting emails I send. Posting this query as I'm short on knowledge. 

I'm using MB Privacy 1.8.5.

Thank you,

Doug

[treed]

This is not in any way related to Privacy, or any other VPN. It sounds like you have a certificate that you're using for signing your e-mail, but it may have expired or otherwise no longer be considered valid, perhaps due to changes in the list of trusted root certificates in the macOS update. In such a case, telling your system to trust the certificate won't do much good, because the recipients' systems may not recognize the validity of the certificate in these situations either, and they will be unable to validate the signature on your e-mails.

I can't give more specific guidance than that, based on the information provided.

[Johnsond72]

Thank you, Thomas. Appreciate your thoughts. Helps rule out several possibilities. As I had no reason, nor any inclination to root around in esoteric email settings, another possible suspect for the new behavior is the macOS update. Casual searching so far has not found any other reports of issues like mine. Finally, and perhaps more likely, as a user of AT&T email, I have seen their struggle to stay afloat. The telling sign is seen in the URL now used: "currently.att.yahoo.com"! Looks more like a floating crap game than a stable operation. 

Thanks for your prompt and thorough reply.

Doug