IMAP Email creates winmail.dat attachment on cell phone

Question

One of my clients is experiencing a very weird situation. We switched his email account from one ISP to another. Before we deleted his account, we copied all his emails to a local folder on his PC. After we created the new IMAP email account on his PC we restored the old emails. The emails then synced with the server and we could see all the emails that we restored on the PC is now on the server together with the pdf attachments.

However, when the SENT Folder synced to his cell phone, the pdf files were gone and replaced with a winmail.dat file. This has only happened with the old emails that were backed-up and restored. All new emails that is send from the PC sync correctly to the server and to his cell phone.

What would cause the old emails to loose the pdf attachment when the emails synced from the server to his phone, and also why only the old emails.

More importantly how can this be fixed please.