Disappearing email.

[#1]

We have a home network comprising a desktop and  laptop.
Both computers use Windows 7 with Windows Live Mail 2011 as default email accounts.
When my wife switched on her laptop this morning she noticed an email subject addressed to me. She did not open the email but almost immediately the email disappeared and now cannot be found on laptop or desktop. I checked Plusnet Webmail and it was not there either despite our settings being, 'Remove from server when deleted from deleted items.'
I checked with the originator of the email and, yes he did send me an email the previous night.
There is no way the email could have been deleted by mistake.
I know that it is near impossible to successfully synchronise one email account on two separate computers and sometimes one email will only appear on one computer; however I have never experienced this particular issue.
I shall be most grateful for any ideas!

[#2]

I see no-one else has responded, so I don't know if any of my thoughts may be of help - I don't use Windows Live Mail 2011 (nor Windows 7) so that may or may not be considered to be a good start  
I'm assuming that the setup of Windows Live Mail is broadly not too dissimilar to Outlook/Outlook Express and that you are using IMAP for the mail rather than POP, in which case using 2 (or more) computers shouldn't be an issue as long as the users are aware that if one users deletes a message then the others will not see it. If that may be an issue then I'd change the setting 'Remove from server when deleted from deleted items' to Not Delete, so periodically you may go to the Deleted Items folder on the Server, check through and then delete those you definitely don't want.
Whilst you say that there is no way the email could have been deleted by mistake, it can be so easy to make the wrong click or click delete on the wrong item, I'd make that server change just in case. Also check in your client for a setting that "Deletes mail immediately" or similar, rather than just putting it in the Deleted Items folder where you then have to explicitly go and delete the lot (or one by one if you choose).
Hope that's of some help or points you in the right direction. Also try http://www.plus.net/support/email/setup/

[#3]

Many thanks for your reply.
The strange thing about this incident is that when my wife opened Windows Live Mail and looked for  new emails she saw my email subject for just a few seconds and even if she had wanted to she couln't open it in time before it disappeared completely.
Our email settings place any deleted emails into the Deleted Items folder where they are held until we decide to delete them forever. Likewise, any spam emails are held in the Junk Mail folder until they are reviewed and deleted as necessary.
There is no way my wife could have deleted that email by mistake.
There was nothing running in the laptop, ie antivirus updates, Windows updates that could perhaps have caused the issue.
I shall certainly take your advice and change the server settings to 'Do not delete' in case the issue reoccurs.
Thanks again for your reply.