As a matter of local policy, user names don't in fact contain bad
characters, but it seems good to be careful anyway.
${if def:sender_address \
{(envelope-from $sender_address\
${if def:authenticated_id \
${if def:sender_address \
{(envelope-from $sender_address\
${if def:authenticated_id \
- {; auth=$authenticated_id}})\n\t}}\
+ {; auth=${quote_local_part:$authenticated_id}}})\n\t}}\
id $message_exim_id\
${if def:received_for {\n\tfor $received_for}}
id $message_exim_id\
${if def:received_for {\n\tfor $received_for}}