As if they weren't an inefficient enough organisation to deal with in other respects, today a division of British Gas asked me to send them a remittance advice to:

CardiffC&[email protected]

Yes, that really is an ampersand in the local part. Sufficiently unusual that trying to send to it upset my default-configured installation of Exim:

rejected RCPT : restricted characters in address

You can tone down the (perfectly reasonable) check for these iffy characters by exempting from it: edit /etc/exim4/conf.d/acl/30_exim4-config_check_rcpt and edit the domains line of the second 'restricted characters in address' ACL to read:

domains = !+local_domains : !