Question : connection to Exchane server via different Internet service providers

Hi all, we have Exchange 2003 and external users use Outlook 2003 with RPC over HTTP for connection, sometimes OWA. The problem with different Ineternet Serveice Providers, for ISP_1 all connection work perfect, but with ISP_2 works only OWA, for Outlook 2003 it shows that the server is not available. Unfortunately we can not change ISP_2 and we have to fix this problem, please advise what should I say to ISP_2 for check/change/fix for to be able to connect external Outlook client to Exchane server.
Appreciate any suggestion

Answer : connection to Exchane server via different Internet service providers

JimboEfx, thanks for your advices and suppport.
the problem was solved by manualy instalation of the certificate of our mail server on notebook, now it works.
But i don't understand how and  why it works before via ISP_1??? anyway thanks again