Problem

quoteaddr() formats addresses for SMTP MAIL FROM: and RCPT TO: commands, which require angle-bracket format (<addr>). It first tries email.utils.parseaddr(); when that fails (for inputs like '<', '< ', '@'), a fallback path checks if the input starts with < and returns it verbatim — without verifying it also ends with >.

The fallback is intentionally lenient with unparseable input (the source comment reads "use it as is and hope for the best"), but a half-open < with no > produces structurally invalid SMTP commands.

Reproducer

from smtplib import quoteaddr
print(repr(quoteaddr('<')))   # '<' — missing closing >

Fix

In Lib/smtplib.py, in the fallback branch of quoteaddr, verify the string also ends with > before returning it. If not, append >:

if addrstring.strip().startswith('<'):
    if addrstring.strip().endswith('>'):
        return addrstring
    return addrstring.strip() + '>'

• Issue: smtplib.quoteaddr() returns malformed angle-bracket address for input '<' #145552