Since any number of levels is possible within the domain hierarchy, specification of a fully qualified address can become inconvenient. This standard permits abbreviated domain specification, in a special case:
This feature is subject to approval by local sub-domains. Individual sub-domains may require their member systems, which originate mail, to provide full domain specification only. When permitted, abbreviations may be present only while the message stays within the sub-domain of the sender.
Use of this mechanism requires the sender's sub-domain to reserve the names of all top-level domains, so that full specifications can be distinguished from abbreviated specifications.
For example, if a sender's address is:
and one recipient's address is:
and another's is:
then ".registry-1.organization-X" need not be specified in the the message, but "registry-C.registry-2" DOES have to be specified. That is, the first two addresses may be abbreviated, but the third address must be fully specified.
When a message crosses a domain boundary, all addresses must be specified in the full format, ending with the top-level name-domain in the right-most field. It is the responsibility of mail forwarding services to ensure that addresses conform with this requirement. In the case of abbreviated addresses, the relaying service must make the necessary expansions. It should be noted that it often is difficult for such a service to locate all occurrences of address abbreviations. For example, it will not be possible to find such abbreviations within the body of the message. The "Return-Path" field can aid recipients in recovering from these errors.