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When installing the Dovecot IMAP Server on Ubuntu its certificate is only valid for a short time.
You can create a new certificate by issuing:

openssl genrsa -out server.key 1024
openssl req -new -x509 -key server.key -out server.pem -days 1826
sudo mv server.key /etc/ssl/private/ssl-cert-snakeoil.key
sudo mv server.pem /etc/ssl/certs/ssl-cert-snakeoil.pem
sudo /etc/init.d/dovecot restart

But take care that you specify the server’s fully qualified domain name (the name you use for connecting) as the common name ! Otherwise Mozilla Thunderbird will not allow you to accept that certificate permanently. However we will still need to accept that certificate manually since it’s self-signed.