I thought this could be useful for other Namecheap customers.
'Namecheap’s Open-Xchange servers use only
'secure' connnections (STARTTLS, SSL/TLS). Supposing
domain.com is the domain you have purchased, you’ll have to configure Thunderbird using these settings:
That should do the work. I use POP3 with SSL/TLS (Port: 995). Be sure you use
'Normal password' as
authentication method. Voila !