3.4.6. BRACKETING CHARACTERS

There is one type of bracket which must occur in matched pairs and may have pairs nested within each other:

• Parentheses ("(" and ")") are used to indicate comments.

There are three types of brackets which must occur in matched pairs, and which may NOT be nested:

• Colon/semi-colon (":" and ";") are used in address specifications to indicate that the included list of addresses are to be treated as a group.

• Angle brackets ("<" and ">") are generally used to indicate the presence of a one machine-usable reference (e.g., delimiting mailboxes), possibly including source-routing to the machine.

• Square brackets ("[" and "]") are used to indicate the presence of a domain-literal, which the appropriate name-domain is to use directly, bypassing normal name-resolution mechanisms.