Which RFC defines the mailto URI scheme used for email links?

The RFC that defines the mailto URI scheme used for email links is RFC 6068.

RFC 6068, titled “The ‘mailto’ URI Scheme,” specifies how a mailto: link represents an email address and optional header fields. A link can identify one or more recipients and can prefill fields such as subject or cc. For example, a website may use a mailto link to open the visitor’s configured email client with an address and subject already supplied.

RFC 6068 is the current refinement in the scheme’s standards history, not its first appearance. RFC 1738 originally defined the mailto URL scheme in 1994, and RFC 2368 expanded it in 1998. RFC 6068 updated and obsoleted RFC 2368 in 2010, reflecting the transition from the older URL terminology to the broader URI framework and clarifying syntax and encoding rules.

RFC 3986 is the general URI syntax specification, while RFC 5322 defines the Internet message format and ordinary email headers. Neither is the dedicated specification for mailto. RFC 6068 also discusses percent-encoding and security concerns such as address harvesting from publicly visible links.