sympl-mail: Exim deny-unusual-characters acl is a little over-strict for outgoing mail.
Non-local domains deny
! in email addresses, although they're valid, and it seems like Xero are using
!'s in emails in some cases.
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.