Email Message "Pending Send", Emails Duplicating

Hello,

We have an issue whereby any emails sent from CRM that contain an attachment are stuck with a status "Pending Send". The strange issue though is that the emails are actually sent but remain as "Pending Send". This is causing the emails to be duplicated each time that CRM tries to resend them.

This is happening for all users and never used to be an issue. Nothing has changed on this environment.

CRM 2016 On Premise 8.1.0.359

Server-Side Sync (Exchange 2013 On Premise)

Any help greatly appreciated.

Kind Regards

Charlie