[1] FTPメールの日時形式

[2] RFC 524 - Proposed Mail Protocol, 2021-01-31T11:00:07.000Z, 2021-03-10T05:27:36.878Z https://tools.ietf.org/html/rfc524

[3] RFC 539 - Thoughts on the mail protocol proposed in RFC 524, 2021-01-31T11:13:10.000Z, 2021-03-10T05:30:45.819Z https://tools.ietf.org/html/rfc539

[4] RFC 555 - Responses to critiques of the proposed mail protocol, 2021-02-01T01:59:11.000Z, 2021-03-10T05:31:39.630Z https://tools.ietf.org/html/rfc555

[5] RFC 561