[Users] "SSL/TLS certificate is expired"

[Adrian]

I have a server certificate authenticated by LetsEncrypt which
auto-renews every three months.

It did this at the beginning of the month and the new expiry date is
July 2nd.  The private key is is copied to my mailserver's config
directory and is readable by it.

But Claws Mail is complaining that the certificate has expired.

~/.claws_mail/certs contains myserver.com.143.cert and
myserver.com.143.cert.chain.  *.cert.chain has a current timestamp, but
*.cert's is Jan 30th, obviously from the expired certificate.

I assume that on connection, Claws should request the public key from
the server and store it in this form.  

Why does it apparently still think the cert has expired and how do I
stop this happening?

Thanks

Adrian