Emails disappearing

One of our users is seeing emails landing but then disappearing from her inbox almost immediately, and in its place she sees a notification like this:

No other users are affected by this for some reason. Our domain is daisychainprlimited.com - the user's inbox is sarah@daisychainprlimited.com. It behaves the same in both her Outlook and also when we log in to view the inbox on the Zoho website.

This message was created automatically by mail delivery system.

THIS IS A WARNING MESSAGE ONLY.

YOU DO NOT NEED TO RESEND YOUR MESSAGE.

The original message was received at Wed, 15 Nov 2017 21:07:05 +0100
from sarah@daisychainprlimited.com [ sarah@daisychainprlimited.com ]

----- The following addresses had fatal errors -----
[Status: ERROR, Address: < sarahm2704@hotmail.co.uk >, ResponseCode 451,
4.7.500 Server busy. Please try again later from [31.186.226.248]. (AS843)
[VE1EUR02FT034.eop-EUR02.prod.protection.outlook.com]]

Message will be retried for 4 more day(s)

----- Transcript of session follows -----

Received:from DESKTOPQO60KMT (bcde78ca.skybroadband.com [188.222.120.202])
by mx.zoho.eu
    with SMTPS id 1510773715347487.5687172748842; Wed, 15 Nov 2017 20:21:55
+0100 (CET)
Message-ID:<0f8f01d35e46$ff917200$feb45600$@daisychainprlimited.com>
Date:Wed, 15 Nov 2017 19:21:54 -0000
From:"Sarah Milne" < sarah@daisychainprlimited.com >
To:"'Alison Cobb'" < alisoncobb@theambassadors.com >,
    "'Nicola Guy'" < NicolaGuy@theambassadors.com >
Cc:< sarahm2704@hotmail.co.uk >,
    "'Natalie Banyard'" < natalie@daisychainprlimited.com >
Subject:RE: Hello and Ellen Kent
Content-Type:multipart/related;
    boundary="----=_NextPart_000_0F90_01D35E46.FF917200"