AttachmentBlocked is a read-only Boolean property used to indicate if the local file storage for the attachment could not be created. AttachmentBlocked is set to True in the FinishTempStream method when the file indicated in StoredPathName has not been created. 

This condition can indicate that local file creation was blocked by a virus scanner. It could also indicate any condition that prevents creation of the local storage for the file-based attachment. 

Use TIdMessage.ExceptionOnBlockedAttachments to indicate if an exception is raised when an attachment file has been blocked.

FinishTempStream 

TIdMessage.ExceptionOnBlockedAttachments