email.headerregistry.Address fails to quote usernames with leading dots

[augusto-rehfeldt]

Bug report

Bug description:

Documented behaviour: Address.init docstring: "The addr_spec and the string value of the object are both quoted according to RFC5322 rules, but without any Content Transfer Encoding."

Expected: Address(username='.a', domain='example.com').addr_spec returns '".a"@example.com', which parses with username='.a' and domain='example.com'.

Actual: addr_spec returns '.a@example.com'; parsing it raises InvalidHeaderDefect.

import re
from email.headerregistry import Address

u, d = ".a", "example.com"
try:
    if not re.fullmatch(r"[A-Za-z.]+", u) or d != "example.com":
        raise ValueError("input is outside the documented test case")
    # username is supplied unquoted, not as an addr_spec; dots are legal
    # quoted-string content. Only dot-atoms may remain unquoted here.
    local = u if re.fullmatch(r"[A-Za-z]+(?:\.[A-Za-z]+)*", u) else f'"{u}"'
    expected = (local + "@" + d, u, d)
    a = Address(username=u, domain=d)
    if (a.username, a.domain) != (u, d):
        raise ValueError("constructor did not accept the input unchanged")
    s = a.addr_spec
    try:
        b = Address(addr_spec=s)
        actual = (s, b.username, b.domain)
    except Exception as e:
        actual = (s, "parse error", type(e).__name__, str(e))
    if actual != expected:
        print("REFUTATION CONFIRMED:", (u, d), "actual:", actual, "expected:", expected)
    else:
        print("REFUTATION REJECTED: output is correctly quoted and round-trips")
except Exception as e:
    print("REFUTATION REJECTED:", type(e).__name__, str(e))

Output on Python 3.14.6 (Windows-11-10.0.26220-SP0), standard library email.headerregistry:

REFUTATION CONFIRMED: ('.a', 'example.com') actual: ('.a@example.com', 'parse error', 'InvalidHeaderDefect', 'local-part is not dot-atom, quoted-string, or obs-local-part') expected: ('".a"@example.com', '.a', 'example.com')

This report was found and written by an automated property-testing tool I run (bugforge). The reproducer above was executed and its output is pasted unedited; no person reviewed the report before it was filed. The search script is in https://github.com/augusto-rehfeldt/bugforge-results/tree/main/email.headerregistry-20261003-021509-c1

CPython versions tested on:

3.14

Operating systems tested on:

Windows

[ZeroIntensity]

See #158748 (comment).