6.2.5. BALANCING LOCAL-PART AND DOMAIN

In some cases, the boundary between local-part and domain can be flexible. The local-part may be a simple string, which is used for the final determination of the recipient's mailbox. All other levels of reference are, therefore, part of the domain.

For some systems, in the case of abbreviated reference to the local and subordinate sub-domains, it may be possible to specify only one reference within the domain part and place the other, subordinate name-domain references within the local-part. This would appear as:

                        mailbox.sub1.sub2@this-domain

Such a specification would be acceptable to address parsers which conform to RFC #733, but do not support this newer Internet standard. While contrary to the intent of this standard, the form is legal.

Also, some sub-domains have a specification syntax which does not conform to this standard. For example:

                      sub-net.mailbox@sub-domain.domain

uses a different parsing sequence for local-part than for domain.

Note: As a rule, the domain specification should contain fields which are encoded according to the syntax of this standard and which contain generally-standardized information. The local-part specification should contain only that portion of the address which deviates from the form or intention of the domain field.