Spamming Problem

Hello,

I have a mail server, exchange 5.5. I am using the OUTLOOK WEBACCESS FEATURE to my Internet Information Server.

I think this exposes to the internet my corporate email addresses because many email addresses started receiving spamming from the same source.

1. How can I protect my email addresses from being exposed to the Internet through this OUTLOOK WEB ACCESS feature?

2. OK! Now I got spam. Are there any tools for Microsoft Exchange Server 5.5, in order to block the spammer.Also are there any other companies that can report the spammers ?

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Thanks in advance.
Day8th