I know this is a old post, but after having problems myself with airvpn and thunderbird, and being a old UPC/Chello ISP client.   last year ISP Ziggo took over UPC/Chello and now all old clients are ziggo users...but, you can still use your old mail adresses (username@chello.nl, username@upcmail.nl) which is a good thing.   I did the following:     settings/serversettings:   incoming mail (getting email)   pop.ziggo.nl port:995 SSL/TLS and use your normal username and password   outgoing (sendin