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:

; Username  
: your email address (@domain.com)  
; Password  
: password you have set in the Namecheap configuration menu  
; Incoming/outgoing servers  
: oxmail.registrar-servers.com  
; Incoming server type  
: IMAP or POP3  
; Outgoing server (SMTP)  
: 465 port for SSL, 25 or 26 for TLS  
; Incoming server (IMAP)  
: 993 port for SSL, 143 for TLS  
; Incoming server (POP3)  
: 995 port for SSL 110 for TLS

That should do the work. I use POP3 with SSL/TLS (Port: 995). Be sure you use 'Normal password' as authentication method. Voila !