Problem with sending email with attachments to some recipients

This is a strange one. Recently I upgraded a site from imap email to Office 365 Exchange. The clients use Office 2010. One user is having an issue where he has about 6 people he sends email with Excel and PDF attachments from Outlook and they get the email but no attachment.
Everyone else he sends to work fine. All these people are from different domains. he has sent me the email and it works fine

If he sends the email from www.portal.office.com  mail it sends the email and the attachment fine to those people. If he sends from his iPad it also works fine

What could it possibly be and how do you diagnose this?

Thanks
Andrew
Andrew MorrisAsked:
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Jackie ManIT ManagerCommented:
First of all, check whether the user saves the email address of contacts in Contact of Outlook 2010. In most cases, the user will not maintain the Contact and just depends on the Suggested Contacts. Sometimes, Outlook 2010 sets the Outlook Properties as "Send using Outlook Rich Text Format" which causes the missing attachment or  a winmail.dat problem.

Source: https://www.experts-exchange.com/questions/27778217/O365-and-winmail-dat-file.html
Vasil Michev (MVP)Commented:
I'd also make sure that no add-ins are interfering. Either try to reproduce the issue via OWA or run Outlook in safe mode (hold the CTRL key while launching it or type 'Outlook /safe' in the run box). Same goes for any AV and other types of scanning software - disable them temporary and try to repro.
James EdwardsCommented:
If he 'sends from his ipad'.... 3 quick fire questions 1st...
FROM the same Exchange account?  
TO the same recipients?
When sending from Outlook client, the e-mail arrives, but just without the attachment, correct?
Andrew MorrisAuthor Commented:
Hi,
I finally resolved this problem bu deleting the autocomplete email address and also the contact from "Contacts"
I then manually added the contact back to "Contact" and when I sent a new email with attachment to the end user they received it.
The client was using Windows Live Mail and this for some reason was stripping the attachment. The email would arrive to there POP account on the web client.

Thanks for all your help

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Andrew MorrisAuthor Commented:
Just took a matter of time to pick through the issue and sort out
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