sympl-mail: Exim deny-unusual-characters acl is a little over-strict for outgoing mail.
Non-local domains deny %
and !
in email addresses, although they're valid, and it seems like Xero are using !
's in emails in some cases.
Replacing local_parts = ^[./|] : ^.*[@%!] : ^.*/\\.\\./
with local_parts = ^[./|] : ^.*@ : ^.*/\\.\\./
in https://gitlab.mythic-beasts.com/sympl/sympl/-/blob/buster/mail/exim4/sympl.d/10-acl/50-acl-check-rcpt/20-deny-unusual-characters should fix this.