I think tyhis was because the front end services were retsarting due to a certificate swappover in Exchange Powershell
Log Name: Application
Source: MSExchangeFrontEndTransport
Date: 20/02/2023 16:45:21
Event ID: 1035
Task Category: SmtpReceive
Level: Warning
Keywords: Classic
User: N/A
Computer: server.domain.local
Description:
Inbound authentication failed with error LogonDenied for Receive connector Default Frontend SERVER. The authentication mechanism is Login. The source IP address of the client who tried to authenticate to Microsoft Exchange is [73.110.121.109].
Event Xml:
<Event xmlns="http://schemas.microsoft.com/win/2004/08/events/event">
<System>
<Provider Name="MSExchangeFrontEndTransport" />
<EventID Qualifiers="32772">1035</EventID>
<Level>3</Level>
<Task>1</Task>
<Keywords>0x80000000000000</Keywords>
<TimeCreated SystemTime="2023-02-20T16:45:21.695446900Z" />
<EventRecordID>8038794</EventRecordID>
<Channel>Application</Channel>
<Computer>server.domain.local</Computer>
<Security />
</System>
<EventData>
<Data>LogonDenied</Data>
<Data>Default Frontend SERVER</Data>
<Data>Login</Data>
<Data>73.110.121.109</Data>
</EventData>
</Event>
