# SendMail fails to send zipped folder

I have run into a problem sending mail from within Mathematica. I can send attachments of most file types: .jpg, .nb, .xls, etc. I have run into a problem trying to send a zipped folder. I have looked for an explanation both here and on Gmail help, but have not been able to determine the cause of the problem. Any insight would be appreciated. Shown below are an example of an operation that works and an operation that fails.

The following works fine:

SendMail[
"To" -> "gandalf@gmail.com",
"Subject" -> "Example of email sent from Mathematica",
"Body" ->
"Hi.. I am testing something. There should be a picture attached.",
"Attachments" -> "picture.JPG",
"From" -> "frodo@gmail.com",
"Server" -> "smtp.gmail.com",
"PortNumber" -> 587,
"EncryptionProtocol" -> "StartTLS"]

{"MessageID" -> "28093986415665034269.14.WolframLanguage.frodo@gmail.com"}


Whereas this fails:

SendMail[
"To" -> "gandalf@gmail.com",
"Subject" -> "Example of email sent from Mathematica",
"Body" ->
"Hi.. I am testing something. There should be a zipped folder attached.",
"Attachments" -> "stuff.zip",
"From" -> "frodo@gmail.com",
"Server" -> "smtp.gmail.com",