RFC Errata
RFC 6068, "The 'mailto' URI Scheme", October 2010
Source of RFC: IETF - NON WORKING GROUPArea Assignment: app
Errata ID: 4706
Status: Reported
Type: Technical
Publication Format(s) : TEXT
Reported By: stream9
Date Reported: 2016-06-08
Section 2.3 says:
3. Whitespace and comments within <local-part> and <domain> MUST NOT be used. They would not have any operational semantics.
It should say:
3. Whitespace and comments within <local-part> and <domain> MUST NOT be used except quoted whitespace in <quoted-string>. They would not have any operational semantics.
Notes:
<local-part> contain <quoted-string> and <quoted-string> contains <quoted-pair> according to definition in RFC 5322.
quoted-pair = ("\" (VCHAR / WSP)) / obs-qp
As definition above, <quoted-pair> contains whitespace <WSP> which according to RFC 6068, MUST NOT be used.
But example in RFC 6068 section 6.2 contain quoted whitespace. So I guess this is an exception.