How to generate a persistent TLS Fingerprint for the msmtp SMTP client

I am using msmtp as a mail client on an M1 iMac still running Monterey. I use Gmail as a relay. It's been working fine, except recently I started getting the following error message:

send-mail: TLS certificate verification failed: the certificate fingerprint does not match

So, I generate a new fingerprint with this:

msmtp --serverinfo --tls --tls-certcheck=off --host=smtp.gmail.com --port=587 | egrep -o "([0-9A-Za-z]{2}:){31}[0-9A-Za-z]{2}"

And then it works, until it doesn't. In the last couple of days I've had to generate three new fingerprints. Why do the fingerprints seem to expire and is there anything I can do about it?