Exchange Help

I have some questions about Microsoft Exchange and how it works.

I have a 2003 Exchange server which works fine internally but when people are offsite they are unable to send or receive emails.

I have to have an exchange account and a pop/smtp account configured for everyone or they do not receive all of their emails.

The real problem is that my customers are unable to use their outlook client from offsite and have to use our provider-supplied webmail option – which usually excludes any mail sent via exchange.

Any advice?