I'm running postfix, dovecot, apache, php, mysql etc on Ubuntu 12.04 and (I think) I now need to reset my hostname and regenerate my smtpd.cert SSL certificate so that my certificate is validated by email clients. At the moment, Thunderbird launches a warning dialog saying "Wrong site: Certificate belongs to a different site" and prompting user to cancel or create a security exception.

So, I guess I need to change hostname to reflect my primary domain, then generate a new SSL cert with those same details.

Is this correct, and what other unforeseen impacts could changing the hostname have?

From what I could find, I need to change hostname in "/etc/hostname" and "/etc/hosts", then "sudo service hostname restart" and also change hostname for postfix in "/etc/mailname"

Anywhere else?

Postfix identifies itself according to the value set for 'myhostname' in main.cf

If myhostname is not set, the fqdn of the server is used.

So you could set myhostname to agree with your cert....

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sleddog