You don't say what version of Outlook your using, but MS have steadily been increasing the number of attachments that re blocked by default, at the last count this was up to 72.
This MS Knowledge base article might help
http://support.microsoft.com/kb/829982 Personally I use
Thunderbird and in the past it's the rest of the Netscape/Mozilla family of email clients.
Today Thunderbird is a capable of email, RSS reader and Usenet client. It can handle multiple POP3 or IMAP accounts and compose/display HTML formatted mail, and best of all it's FREE!