email settings. (General)
These issues might need to be handled outside SQL-Ledger.
Have you enabled SPF and DKIM records in your DNS settings ?
While providing no guarantees, these records may increase your prospects of successful email delivery to your customers.
What mailserver delivers your outgoing emails ? Exim or Postfix perhaps ?
Does your mailserver add DKIM headers to your outgoing emails including Invoices ?
And IF you're also running a public facing webserver, there's other things you can do to confirm email delivery of invoices, statements, etc.