We use an off the shelf product at work which can send e-mails. The e-mails come from the currently logged in user and the way this is achieved is by using Exchange Web Services. The code probably looks something along the lines of;

ExchangeService service = new ExchangeService();
service.Credentials = new WebCredentials("impersonationusername@domain.com", "password", "");
service.UseDefaultCredentials = false;
service.AutodiscoverUrl(from, new AutodiscoverRedirectionUrlValidationCallback(RedirectionUrlValidationCallback));
service.ImpersonatedUserId = new ImpersonatedUserId(ConnectingIdType.PrincipalName, "sender");
EmailMessage email = new EmailMessage(service);
email.Subject = "subject";

Our e-mail provider is Office365 and recently some users have been unable to send e-mails from this product. We followed the usual troubleshooting process and were pretty convinced we had ruled out everything at our end. On top of this we used the Microsoft Remote Connectivity Analyzer (https://testconnectivity.microsoft.com) which was giving the same result;

ExchangeImpersonation SOAP header must not exist for this type of OAuth token

We haven’t made any changes to the mailboxes recently and they were previously working so it must be an issue Microsoft’s end. We logged a ticket and a week of back and forth later we have a solution (via powershell);

1. Set-ExecutionPolicy RemoteSigned
2. $UserCredential = Get-Credential
3. $Session = New-PSSession -ConfigurationName Microsoft.Exchange -ConnectionUri https://outlook.office365.com/powershell-liveid/ -Credential $UserCredential -Authentication Basic -AllowRedirection
4. Import-PSSession $Session
5. New-MoveRequest -Identity "impersonationusername@domain.com"

You can check the status of the execution of this command by:
Get-MoveRequest -Identity "impersonationusername@domain.com"

Microsoft were unable to provide a real reason for this other- but said something along the lines of; “something has become corrupt and moving the impersonation mailbox to a different database causes some form of rebuild”.

I hope this works for you too!