One of my business partners has recently noticed that her emails are getting bounced back when she tries to send me an email.

This the mail she got.

From: Mail Delivery System [mailto:MAILER-DAEMON@s4.abplusz.hu] 
Sent: Wednesday, July 5, 2017 11:01 PM 
To: xxxxxxxxxx@xxxxxxxxxxx.hu 
Subject: Undelivered Mail Returned to Sender 

This is the mail system at host s4.abplusz.hu. 

I'm sorry to have to inform you that your message could not be delivered to 
one or more recipients. It's attached below. 

For further assistance, please send mail to postmaster. 

If you do so, please include this problem report. You can delete your own 
text from the attached returned message. 

The mail system 

<laszlo AT laszlocsontos.com>: Host or domain name not found. Name service 
error 
for name=laszlocsontos.com type=MX: Host not found, try again 
Reporting-MTA: dns; s4.abplusz.hu 
X-Postfix-Queue-ID: 1A98622DB1 
X-Postfix-Sender: rfc822;  xxxxxxxxxx@xxxxxxxxxxx.hu  
Arrival-Date: Wed, 5 Jul 2017 10:04:02 +0200 (CEST) 

Final-Recipient: rfc822; laszlo AT laszlocsontos.com 
Original-Recipient: rfc822;laszlo AT laszlocsontos.com 
Action: failed 
Status: 4.4.3 
Diagnostic-Code: X-Postfix; Host or domain name not found. Name service error 
for name=laszlocsontos.com type=MX: Host not found, try again 

I believe that all the required records (MX, SPF, DKIM) has been set for laszlocsontos.com.

Could you please check the reason why your the other party cannot find your MX servers?

Thanks,

László