GFI MailEssentials blocks files that do not have a file name.
all supported environments
To disable GFI MailEssentials from blocking attachments without a file name, please follow the procedure below:
In GFI MailEssentials this option will be listed under EmailSecurity > Email Exploit Engine
Disable the detection of the exploit Attachments without a file name (suspicious)
Close the GFI MailEssentials configuration
By default the EmailSecurity portion of GFI MailEssentials will block files that do not have a file name. A file that does not have a file extension indicates that the email contains an attachment with an 'application / hta' Content-Type. Such attachments are often undesired.