Same problem here and only with Scarlet, others Pop mails are Ok.
When investigating by doing a telnet on port 110 on pop.scarlet.be and doing the CAPA command, I see that the authentication is : USER SASL PLAIN LOGIN, that seems to cause problem to Lion mail.
When you try the same command on a working Pop server by exemple pop.skynet.be, the CAPA command gives you only USER.
I think the problem come from mail, as the same sequence was well understood under Snow Leopard, and the mail server works find with Thunderbird.
Normally after the CAPA command, it should sends the USER and PASS commands to effectively connect to the mail server, but it's not the case, it just send the QUIT command to end the connection.
The connection is done through a kCFStreamsOcketSecurityLevelNone.
Maybe it might help in the investigations.