# File lib/action_mailer/vendor/tmail/net.rb, line 265
265:     def exec( mail, top = true )
266:       mail.mime_version = '1.0' if top
267:       mail.set_content_type 'multipart', 'mixed'
268:       e = encoding(nil)
269:       if e and not /\A(?:7bit|8bit|binary)\z/i === e
270:         raise ArgumentError,
271:               'using C.T.Encoding with multipart mail is not permitted'
272:       end
273:       mail.parts.each do |m|
274:         exec m, false if m._builtin_multipart?
275:       end
276:     end