It seems the email was being corrupted by AVG antivirus, so if anyone else is having this error try disabling the 3rd party AV solution on the client machine. Disabling/uninstalling AVG did solve the problem.
Here is the log for reference:
Trace: Tracing message with Message ID [795D639E0F4143C99808D95D2590A96E.MAI] from the SMTP Inbound Queue
Result: [795D639E0F4143C99808D95D2590A96E.MAI] was not routed from the SMTP inbound message queue by the local MTA Service.
This may indicate the message was never fully received, or rerouted/deleted by a third party application or filter.
If the server is in a mail cluster, check logs on other servers.
SMTP Debug log matches:
04/30/21 13:37:04 ME-I0070: [795D639E0F4143C99808D95D2590A96E.MAI] (recv) socket [1828] was gracefully closed during [DATA] command by the remote client {HIDDEN}.
Important: Message ID [795D639E0F4143C99808D95D2590A96E.MAI] has not been routed through MailEnable queues. Check the log details for the reason. If in a cluster you will need to check each cluster server log files.
Status: SMTP Inbound transaction details for Message ID [795D639E0F4143C99808D95D2590A96E.MAI] follow below:
04/30/21 13:36:43 SMTP-IN 795D639E0F4143C99808D95D2590A96E.MAI 1828 {HIDDEN} 220 {HIDDEN} ready at 04/30/21 13:36:43 78 0
04/30/21 13:36:43 SMTP-IN 795D639E0F4143C99808D95D2590A96E.MAI 1828 {HIDDEN} EHLO EHLO {HIDDEN} 250-{HIDDEN}[{HIDDEN}], this server offers 7 extensions 269 15
04/30/21 13:36:43 SMTP-IN 795D639E0F4143C99808D95D2590A96E.MAI 1828 {HIDDEN} AUTH AUTH LOGIN 334 VXNlcm5hbWU6 18 12
04/30/21 13:36:43 SMTP-IN 795D639E0F4143C99808D95D2590A96E.MAI 1828 {HIDDEN} AUTH {blank} 334 UGFzc3dvcmQ6 18 26 {HIDDEN}
04/30/21 13:36:43 SMTP-IN 795D639E0F4143C99808D95D2590A96E.MAI 1828 {HIDDEN} AUTH {blank} 235 Authenticated 19 14 {HIDDEN}
04/30/21 13:36:43 SMTP-IN 795D639E0F4143C99808D95D2590A96E.MAI 1828 {HIDDEN} MAIL MAIL FROM:<{HIDDEN}> 250 [SMTP:{HIDDEN}] connection from {HIDDEN} authenticated. 74 30 {HIDDEN}
04/30/21 13:36:43 SMTP-IN 795D639E0F4143C99808D95D2590A96E.MAI 1828 {HIDDEN} RCPT RCPT TO:<{HIDDEN}> 250 Requested mail action okay, completed 43 35 {HIDDEN}
04/30/21 13:36:43 SMTP-IN 795D639E0F4143C99808D95D2590A96E.MAI 1828 {HIDDEN} DATA XSAVETOSENT 354 Start mail input; end with <CRLF>.<CRLF> 46 13 {HIDDEN}