EXIM: to Accept mail from single non existing domain

Hello

I am running my own mail server without any issue, apart from one.
My employer is sending my payslip from a non existing domain :(

So I can reproduce this with a telnet test (in bold is what I sent to the smtp port of my mailserver)
Code:Trying 1.2.3.4 ...
Connected to smtp.mymaildomain.com
Escape character is '^]'.
220 smtp.mymaildomain.com ESMTP Exim 4.96.2-12-g29d01ae2a Thu, 23 Nov 2023 15:30:20 +0100
EHLO nonexistingsubdomain.existingdomain.com
250-smtp.mymaildomain.com Hello myown.hostname.com [4.3.2.1]
250-SIZE 52428800
250-8BITMIME
250-PIPELINING
250-PIPECONNECT
250-AUTH PLAIN LOGIN
250-STARTTLS
250 HELP
MAIL FROM:<payroll@nonexistingsubdomain.existingdomain.com>
250 OK
RCPT TO:<my@mmymaildomain.com.com>
550-Verification failed for <payroll@nonexistingsubdomain.existingdomain.com>
550-Unrouteable address
550 Sender verify failed
Terminatedand obviously in my eximlog I see:
Code:2023-11-23 15:30:20 H=myown.hostname.com [1.2.3.4] sender verify fail for <payroll@nonexistingsubdomain.existingdomain.com>: Unrouteable address
2023-11-23 15:30:20 H=myown.hostname.com [1.2.3.4] X=TLS1.3:TLS_AES_256_GCM_SHA384:256 CV=no F=<payroll@nonexistingsubdomain.existingdomain.com> rejected RCPT <my@mymaildomain.com>: Sender verify failed
2023-11-23 15:30:20 H=myown.hostname.com [1.2.3.4] incomplete transaction (QUIT) from <payroll@nonexistingsubdomain.existingdomain.com>so it fails for good reasons, cause nonexistingsubdomain.existingdomain.com is indeed a nonexisting subdomain

I cannot get my employer to change the hostname or create a dns record... I tried and failed, the simply won't listen and follow best practise

I really only need to accept that mails from payroll@nonexistingsubdomain.existingdomain.com are being bypassed from this check
How would I whitelist that and exempt that from any testing?

I already tried creating a /etc/hosts entry and I also added the hostname to the installed dns server, didn't do a damn thing

any help would be welcome