The question of whether the character “_” is permissible within the local part of an electronic mail address is a frequent one. Officially, the specifications that govern internet email address formats, namely RFC 5322 and its predecessors, allow a wide range of characters, including underscores. Therefore, an address like “john_smith@example.com” is technically valid.
The allowance of these characters is intended to provide flexibility in address creation. Historically, this flexibility has been important for users and organizations needing to create distinctive addresses. However, the practical reality is that not all systems and applications correctly implement the full specifications. Older systems, or those with overly strict validation rules, might reject addresses containing underscores, despite their technical validity. This can lead to communication failures and user frustration. Furthermore, the use of such characters can sometimes be confused with spaces, leading to errors.
