Select Tools -> Options... from the menu.
Go to the Mail Format tab.
Under Compose in this message format:, make sure either HTML or Plain Text is selected.
Click Internet Format.
Make sure either Convert to Plain Text format or Convert to HTML format is selected under When sending Outlook Rich Text messages to Internet recipients, use this format:
Click OK.
Click OK again.

Disable Winmail.dat Stubbornly Going to Particular Recipients No Matter the Default:

Search for the desired contact in your Outlook Contacts.
Double-click the contact's email address.
Alternatively, click on the desired email address with the right mouse button and select Outlook Properties... from the menu.
Make sure either Let Outlook decide the best sending format or Send Plain Text only is selected under Internet format.
Click OK.

When, How and Why Winmail.dat-Application/MS-Tnef is Created

It's Outlook's fault, in a way. Or the recipient's email client's. If Outlook sends a message using the RTF format (which is not very common outside Outlook) for bold text and other text enhancements, it includes the formatting commands in the winmail.dat file. Receiving email clients that do not understand the code therein display it as a stale attachment.

To make matters worse, Outlook may also pack other, regular file attachments in the winmail.dat file.

Fortunately, you can get rid of winmail.dat altogether by making sure Outlook does not even try to send mail using RTF.

1. Open the message in a separate window (double click)
2. Under the View menu select Options
3. Copy the text in the Internet Headers window (unfortunately it doesn't include the message itself).
4. Paste
5. Close the options window

Outlook 2007

1. In main list, click right on the email, where you want to view the header
2. In the list select Message Options
3. Copy the text in the Internet Headers window (unfortunately it doesn't include the message itself).
4. Paste
5. Close the options window

Outlook 2010, Outlook 2012

1. Open the message for which you want to view the headers
2. In the message window, select the File tab
3. Click the Properties button
4. Copy the text in the Internet Headers window (unfortunately it doesn't include the message itself).
5. Paste
6. Close the Properties window

This problem occurs because the Winmail.dat file is used to preserve formatting that the sending client includes in the message, but the receiving client does not recognize the Winmail.dat file. In Outlook, the Winmail.dat file includes Rich Text Formatting (RTF) instructions. This type of formatting is used with the Microsoft Outlook Rich Text format and when you use Microsoft Word as your e-mail editor. Only other Outlook users can read the attachments.

To resolve this problem the person sending the attachments with Outlook needs to do the following:

1. On the Tools menu, click Options and then click Mail Format. 
2.  In Compose in this message format, click to select Plain Text, and then click OK.

 Task 'domain name - Receiving' reported error (0x8004210E) : 'Your mailbox is temporarily unavailable because another e-mail message is being delivered to it or another mail application is accessing it.  The server responded: -ERR Can't get lock.  Mailbox in use'

The Cause: Under certain conditions (eg. network problems, malformed email messages, more than one email client being configured to poll the server) the lock file which is used to stop more than one person logging into the account at once, fails to be deleted at the end of the POP3 session.

It then prevents you from logging in again, generating the Outlook error above.

The Solution: If there is no further activity on the POP3 mailbox, usually within 15-20 minutes it will be unlocked automatically.

However If you are still experiencing this problem contact ProExe support and we will remove the lock file from your account. 

If the problem occurs again, please make sure that you don't have more than one email client configured to check the same account.

This error can also occur if the message being downloading is malformed, and the client computer has anti-virus or anti-spam software running (such as McAfee or Nortons Anti-virus). The software installed on the client has problems with interpreting the email and doesn't finish, and doesn't try to download the next message from the server.

To resolve, the anti-spam or anti-virus software has to be disabled if not at least to diagnose and debug. In some cases a single message could be holding up the download on the client firewall end, once this message is removed or let through by disabling as previously mentioned every thing can be resumed as per the previous configuration settings, and the anti-virus software can be re-enabled.

Choose 'view or change' to change an existing email account.

Select the account to be changed, and click the 'change' button.

Enter the details supplied by ProExe.
Click the 'More Settings' button.

Click 'Outgoing Server' and if required specify that your outgoing SMTP server requires authentication.

If your dial-up or broadband connection requires authentication, tick the box and enter below the necessary authentication method / details.

Now click on the 'Advanced' tab.
Unless advised otherwise, it is normal to use the default settings as shown below.

Click on Okay
Click on Next
Click on Finish