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 !