This is one of those irritations which can arise suddenly, and then finding how to fix it can take a while, so here's the fix!
If you use a fastmail.fm email address and access it via IMAP in The Bat! and get a popup message saying the security certificate is invalid, go to this page in your browser:
http://blog.fastmail.fm/2011/09/02/updated-ssl-certificate-for-messagingnengine-com/ and download the two cert files shown:
https://www.fastmail.fm/certs/DigiCertHighAssuranceEVRootCA.crt and
https://www.fastmail.fm/certs/DigiCertHighAssuranceCA-3.crt - save them somewhere handy to find.
Now go to The Bat! and select the fastmail.fm email account, and right click the account name and select
Properties > General > Edit Personal Certificates Click the
Certificates tab and then the
Import button. Browse to where the downloads certificates you've just saved are and select them both. Once they are imported you'll see them in the Certificates pane. Highlight each one in turn and click
View and then open the
Certification Path tab. Click the
Add to Trusted button, and this will import the new certificate into your Certificate address book. Click
OK to close each pane (there'll be three to work back through.)
Now your fastmail.fm account should connect again without that annoying popup message!
