mail inbox not keeping offline mails

hi - i am new to Macs and been using lion for a couple of weeks now, i have a small problem with the mail client, i always lose all my offline mails as this syncs with my internet account, i have told it to keep copies of mails with attachments for offline viewing but it never does.

Can someone please help me?

Thanks

Without telling us what "mail client" you're using, and who your email provider is, there's no way we can help you.

sorry email provider is btinternet and mail client is apple mail 5.0

thank you

Anyone.