That sounds like an antivirus issue. Outlook uses the easily hacked windows security store which antivirus vendors routinely register themselves as authorized certificate issuers without any form of oversight. Thunderbird offer no simply hack for certificates. The acceptance in Thunderbird is a manual process and therefore you have to follow some procedure the antivirus vendor has to add their certificate to the store. Personally I think allowing the antivirus to read all my encrypted messages is a greater risk than having no scanning (so I use a product that does not scan email). But that is my personal choice.