X.400 is the Message Handling Service Protocol for e-mail transmissions specified by the ITU-T and ISO. X.400 is common in Europe and Canada and is an alternative to the more popular e-mail protocol, Simple Mail Transfer Protocol (SMTP), which is defined by IETF. X.400 uses binary format so it is easy to include binary contents without encoding it for transfer. Also it is harder for people to fake email addresses and contents, compared with the situation in STMP where text messages are used.
X.400 and STMP have similar features but also unique features in themselves. Generally speaking, X.400 is a more complex protocol with the following features that are not in the SMTP:
- Delivery notifications - Delivery notifications are used both about delivery notifications (yes, the message got here) and about non-delivery notifications (no, the message did not get there).
- Receipt notifications - A receipt notification is passed back to the originating user indicating that what happened to the message after it was delivered (for instance that it was read by the recipient). In X.400, receipt and non-receipt notifications may include notifications of something being automatically forwarded, messages deleted, etc.
- Security functions C X.400 defined a framework for mail transmission securities. It defined the concept of a "security label" and allowed using an OID for identifying your security labelling scheme, but no labelling scheme was actually specified in the protocol.
- Priority makers (3 levels) C this feature is used for ordering the queue of mails to send, so that "important" mails get sent before "less important" mails.
- Deferred delivery C Schedule delivery time for messages. This feature has not been widely deployed.
- Conversion in the network Cconversion works like converting Teletex to plain text, or fax images to text saying "there was a picture here, but you are not allowed to see it". Conversion never improves a message, and it is impossible to support security functions like signatures or encryption while doing conversion in the network.
- Reliable Transfer Service C This X.400 feature gives you the ability to continue transferring a document after transfer was interrupted.
SMTP has some functions that X.400 do not, which include the following:
- Standard functionality to check each recipient for validity before transferring the message; X.400 requires transferring the complete message before checking recipients.
- Optional functionality for checking whether a message is too large to transfer before sending it.
- Ability to insert any data into the headers of a message with a fair probability of it being presented to the user
- Ability (MIME Multipart/Alternative) to send several representations of the same content in the same message, guaranteeing both interoperability with the lowest common denominator and no loss of information between compatible UAs.
In addition, an X.400 address is different with that of STMP. X.400 consists of a set of bindings for country (c), administrative domain (a), primary management domain (p), surname (s), given name (g). An SMTP e-mail address that looks like this hypothetical address: Jeff.warson@javvin.subdomain.us
It looks like this in an X.400 e-mail message:
G=Jeff; S=warson; O=subdomain; OU=javvin; PRMD=attmail; ADMD=attmail; C=US
Protocol Structure
X.400 was designed with attributed addresses. The complete set of attributes is rather large:
| Attribute Type | Abbreviation | Label |
| Given Name | Given name | G |
| Initial | Initials | I |
| Surname | Surname | S |
| Generation Qualifier | Generation | Q |
| Common Name | Common Name | CN |
| Organization | Organization | O |
| Organizational Unit 1 | Org.Unit.1 | OU1 |
| Organizational Unit 2 | Org.Unit.2 | OU2 |
| Organizational Unit 3 | Org.Unit.3 | OU3 |
| Organizational Unit 4 | Org.Unit.4 | OU4 |
| Private Management Domain Name | PRMD | P |
| Administration Management Domain Name | ADMD | A |
| Country | Country | C |
| Physical Delivery Personal Name | PD-person | PD-PN |
| Extension of Postal O/R Address Components | PD-ext. address | PD-EA |
| Extension of Physical Delivery Address Components | PD-ext. delivery | PD-ED |
| Physical Delivery Office Number | PD-office number | PD-OFN |
| Physical Delivery Office Name | PD-office | PD-OF |
| Physical Delivery Organization Name | PD-organization | PD-O |
| Street Address | PD-street | PD-S |
| Unformatted Postal Address | PD-address | PD-A1 |
| (there are individual labels for each line of the address) | PD-A2 | |
| PD-A3 | ||
| PD-A4 | ||
| PD-A5 | ||
| PD-A6 | ||
| Unique Postal Name | PD-unique | PD-U |
| Local Postal Attributes | PD-local | PD-L |
| Postal Restante Address | PD-restante | PD-R |
| Post Office Box Address | PD-box | PD-B |
| Postal Code | PD-code | PD-PC |
| Physical Delivery Service Name | PD-service | PD-SN |
| Physical Delivery Country Name | PD-country | PD-C |
| X.121 Network Address | X.121 | X.121 |
| E.163/E.164 Network Address | ISDN | ISDN |
| PSAP Network Address | PSAP | PSAP |
| User Agent Numeric ID | N-ID | N-ID |
| Terminal Identifier | T-ID | T-ID |
| Terminal Type | T-TY | T-TY |
| Domain Defined Attribute | DDA: | DDA: |
Related protocols: SMTP, ROSE, ACSE, RTSE
Sponsor Source:X.400 protocol is defined by ISO (http://www.iso.org) and ITU-T (http://www.itu.org)
Reference:http://www.itu.int/rec/recommendation.asp?type=products&parent=T-REC-f : X.400 Standards List