sendmail

Im running Fedora Core 6 with sendmail, procmail, openldap and uw-imap  
installed.  Sendmail is configure with openldap for mail and user  
accounts.  Most of my users check their mail through horde and imp  
webmail which works well.  However, a few users use Mozilla  
Thunderbird email client.  When they are inside my network they can  
send and recieve emails no problem.  When they go home they are unable  
to send emails.  The mail client keeps asking for username and  
password.  Im thinking that my pam module for smtp isnt configured  
correctly to grab the username and passwords from my ldap directory.   
But I cant quite put my finger on the issue.  Im not seeing anything  
weird in my logfiles.  I actually tried sending an email from  
thunderbird while watching my secure logs and nothing happens.  But  
Mozilla comes back with wanting username and password again.  Anyone  
using anything close to my mailserver setup?  Im sortof at a loss as  
to why this doesnt work.