Assuming WLMail 2011 or 2012, open WLMail, Alt, F, O, M, Advanced tab, Maintenace button, check Compact the database on every xx closes.

Change the number to one. Close WLMail.

Wait until Task Manager shows that wlmail has closed.

Reopen WLMail and change the number of closes back to 100.

WLMail has indexes for the stored messages. When an index becomes corrupted, all links point to one email.

Causes of this problem:

Norton or other software (tell Norton to not scan .eml files)
Closing WLMail and immediately closing Windows
Moving large amounts of emails

It did not work