I'd say that, if it is an attachment block, it is one of two places:

1. Outlook security settings

2. Your ISP (often they won't tell you about security measures implemented to stop attacks against the ISP itself).

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If it is not an attachment block, it could be:

A. The sender's ISP, especially if it is AOL.

B. Your ISP blocking the sender's ISP.

C. Failure of the sender to attach the file correctly (such as trying to embed some kinds files, rather than trying to attach them).

D. The attachment is larger than your ISP allows an email to be.

E. Spam or spyware filter blocking the email.

F. Virus checker detecting a virus.