Email encryption for most people is sufficient even if the metadata is exposed. One can simply state in their email encryption "Bing Bing Bong" or "Why did you not put the trash out?" which might mean to the recipient :: "check the second SFTP server" or "let the cat outside" or "Jump on my private Mumble chat server" or "Get on my private self hosted IRC server". The email message need not be encrypted for that matter.

The intended payload can be in an header-less encrypted file on a throw-away SFTP server in the tmpfs ram disk.