When you receive an e-mail from an address that doesn’t end in @gmail.com, Gmail looks at attachments for file extensions known to be executable (such as .dll, .exe, .vbs, and so forth), so if someone sends you one of these file types, their message will bounce back. This goes for files within ZIP archives as well—Gmail looks inside these for executable extensions and the e-mail bounces back to the sender if it contains any. Gmail doesn’t look inside other archive formats, such as RAR or ACE, so you might want to use one of these formats instead of going through the hassle of the following workaround. To get around this annoyance, you can use the same trick that has been used for years. Simply tell the sender to rename the xtension of the file to something Gmail will allow (such as .jpg), and when you receive the file, rename it back to the type it really is (for example, change file.jpg to file.exe). It seems that Gmail will allow you to send and receive executable attachments between Gmail accounts and from Gmail to outside accounts.