Thunderbird doesn't disable security software. We may well ''ask ''you to exclude Thunderbird's profile from your security software's scan, but we don't unilaterally disable other programs. Does ''skymesh.com.au'' seem familiar to you? is this your ISP or email provider? Such messages are common in Thunderbird, particularly where the name stored in a certificate is different from the name offered by the email server itself. Large, well-known or well-run email providers know how to configure their email systems correctly to avoid this. Smaller ISPs and email providers aren't always so smart. In exceptional cases, particularly where using a public wifi hotspot, it is possible that such a message might indicate that someone has hijacked your connection. In all probability, ''you ''are most likely best placed to judge if this warning message is a cause for concern or not. I would view the certificate, and then my email settings and the server addresses used in email headers. If the name in the certificate didn't match the name used in the email account settings or headers, I'd use whois and similar to check that they both belong to the same business. I had a situation where the email certificate used "mail." and the server said "cluster1." (or the other way round; I really can't remember). But I was able to confirm that both had IP addresses pointing to my ISP, and on that basis, accepted the certificate.