The "mail-server" access-type indicates that the actual body is available from a mail server. The mandatory parameter for this access-type is:
SERVER -- The email address of the mail server from which the actual body data can be obtained.
Because mail servers accept a variety of syntaxes, some of which is multiline, the full command to be sent to a mail server is not included as a parameter on the content-type line. Instead, it is provided as the "phantom body" when the content-type is message/external-body and the access- type is mail-server.
An optional parameter for this access-type is:
SUBJECT -- The subject that is to be used in the mail that is sent to obtain the data. Note that keying mail servers on Subject lines is NOT recommended, but such mail servers are known to exist.
Note that MIME does not define a mail server syntax. Rather, it allows the inclusion of arbitrary mail server commands in the phantom body. Implementations must include the phantom body in the body of the message it sends to the mail server address to retrieve the relevant data.
It is worth noting that, unlike other access-types, mail-server access is asynchronous and will happen at an unpredictable time in the future. For this reason, it is important that there be a mechanism by which the returned data can be matched up with the original message/external-body entity. MIME mailservers must use the same Content-ID field on the returned message that was used in the original message/external-body entity, to facilitate such matching.