Has the client not been able to send you back the email with the strike-out?
Could be several things causing this and its possible the problem is most likely not on your end.
What I would do is eliminate your self as the root of the problem
Send the email to your self any other colleagues and another external email account.
Test to see that all emails are being sent and received okay.
Once you have eliminated yourself as the cause of the issue, would then turn it back over to the customer/client.
If the problem is found to be on their end you have to ask yourself:
How much time do you want to spend in trying to figure out someone elses issues, which you are not on site for, and the cause could be anything?
If its a really, really important client, you could go through the lengths of finding out what email client(s) they are using, what service pack they are using if using outlook, if all their users view this email the same way or is it just one user, if they are using any anti-virus or Spam filters and what they are, is this email still viewed this way if they open Outlook in Safe Mode as an Add-In may cause this (Start/Run/Outlook.exe /safe) or if they have any custom views set-up that may be causing this.
If its a custom view that may be the problem you could have them close and reopen outlook using the /cleanviews switch and this may fix it, but all their other custom views will be lost and they will have to recreate them.
You may be able to help them and you may not. If you are, you have to realize if anything else unrelated happens to the clients email they will be looking back to you for answers and or blame.
If they have any custom views setup,
