Accepts a message object and formats it as a MIME document.
# S3 method for envelope
as.character(x, ...)A message object.
Further arguments passed to or from other methods.
A formatted message object.
There are a number of options for multipart messages:
multipart/mixed <U+2014> Used for sending content with multiple independent parts either inline or as attachments. Each part can have different Content-Type.
multipart/alternative <U+2014> Used when each part of the message is an "alternative" version of the same content. The order of the parts is important: preferred and/or more complex formats should be found towards the end.
Example: A message with both plain text and HTML versions.
multipart/digest <U+2014> Used to send multiple plain text messages.
multipart/related <U+2014> Used when each part of the the message represents a component of the complete message.
Example: A web page with images.
multipart/signed <U+2014> Used when a message has a digital signature attached.
multipart/encrypted <U+2014> Used for a message with encrypted content.
A nice illustration of how some of these relate can be found at https://stackoverflow.com/a/40420648/633961.