Some E-mails are not received in Windows Mail

Using Windows Mail I find that some recipiants are not receiving E-mails I send to them. Even when I reply to their E-mails the reply does not go through to their mailbox. They have checked their Spam inbox, but the E-mail does not show up there either. I would appreciate any help along these lines. Thank you. "-)

Can you see the emails on your Sent folder?

Do those emails contain attachements or just text?
I would check the antivirus program , email account settings, (might require smtp authentication) or the recipient's server settings might be rejecting them.

Can you send to them using another app? e.g. Windows Live Mail?