There are a number of reasons why messages may not arrive in your inbox.

Messages can skip your inbox and end up in your All Mail, Spam, or Trash labels, as the result of automatic spam filtering or filters that you've set up on your messages.

Also, if you're sending mail to a mailing list that you subscribe to, those messages will only appear in 'Sent Mail.' This behavior also occurs when sending to an email address that automatically forwards mail back to your Gmail address.

Please check your Spam and Trash labels, and try performing a search of all mail in Gmail for the message in question. You can search for conversations by entering the sender's name, the subject of the email, or a phrase from the content of the email, if you know it. Click Show search options, located next to the search box on any Gmail page, and make sure that Mail & Spam & Trash is selected in the Search field. Please note that Gmail search results don't include matches to partial strings or matches that are 'similar' to your query, including plurals and misspellings. If you search for vacation, Gmail returns emails containing instances of vacation, but does not return emails including vacations or vaccations.

Hotmail Filters
In addition to the anti-spam filtering technologies, Windows Live Hotmail also enables each user the ability to set filter levels to further improve the delivery of e-mail to their account. Users can easily add a sender or domain name to the Allowed Sender List so that the e-mail from that sender or domain is never treated as junk regardless of the content of the message. Users can configure the settings to accept only messages from the Contacts and Safe Senders List, giving total control over which messages are received.

E-mail messages from a certain e-mail address or domain name can also be easily blocked by adding the sender to your Blocked Senders List. In addition, each time a message is reported as junk using the "Report and Delete" function, messages from those senders or Internet domains are automatically added to a users Blocked Senders List and henceforth will always be treated as junk e-mail messages regardless of the content of the message.

The best thing to do first is check with the sender to make sure that they actually sent the message. You might be surprised how often this simple explanation fits the problem. Then again, you've got smart people sending you email, so...

If they have sent it, here's a list of conditions that will stop that email from getting through to you. If any of them describes your situation, you'll find instructions on how to resolve the issue just below. Hopefully, one of these will do the trick!

Incorrectly typed email address
Is the sender using your correct email address? Just one mistyped letter will cause an email to go to the wrong address or make it undeliverable. Ask them to double-check that they entered your address correctly.

Sender's email not functioning properly
Can this sender successfully send messages to other people? The problem could be on their end, with sending. Ask them to check.

Inbox needs refreshing (Yahoo! Mail Classic)
New email often arrives while you're writing or reading another email. The all-new Yahoo! Mail updates your Inbox for you, so if you use it, you'll always see new mail when it arrives. But in Yahoo! Mail Classic, you won't see this new mail in your Inbox until you Refresh or Reload the page, or return to the Welcome Screen, or click Check Mail.
Note: Yahoo! Messenger shows an alert each time a new email arrives. Helpful!

One of your filters could be directing the email into a folder
If you've set up filters, the message from this sender might be going directly to a different folder from your Inbox. Here's how to check your filters:

• Click Options in the upper-right, and select Mail Options.
• Click Filters.
• See anything that might be capturing the mail you've been looking for?
• Note: Messages in Trash can be deleted at any time. Once messages are deleted, they're gone forever, and can't be retrieved.

• Click Options in the upper-right corner of the page, and select Mail Options.
• Under "Management", click POP Access & Forwarding.
• Is the lower half of the page (Forwarding) selected? If so-and if you would like to discontinue it:
• Click the radio button for "Web and POP Access" at the top of the page.
• Click Save. Done!

• Click Options in the upper-right corner of the page and select Mail Options.
• Under "Spam", click Block Addresses.
• Scan the list for the address of the sender. If you see it, click it, then click Remove Block.

There's a delay in delivery
Messages are usually delivered within a few minutes. Very rarely, they are delayed while in transit. This is usually due to problems on the sender's mail server, heavy Internet traffic, or routing problems. Unfortunately, aside from waiting, there's no way to know whether a message has been delayed or whether it has been rendered undeliverable. If a sender can re-send it, have them do that.

No matter what the cause, the best solution is to ask the sender to re-send the message.

The Junk E-mail Filter in Microsoft Office Outlook 2007 is designed to keep junk e-mail messages, also known as spam, from cluttering your Inbox. By default, the Junk E-mail Filter is turned on, and the protection level is set to Low, the setting designed to catch the most obvious spam. Also by default, any message caught by the Junk E-mail Filter is moved to the Junk E-mail folder. You can change any of these Junk E-mail Filter options for the kind of protection that you want.

NOTE: While the Outlook Junk E-mail Filter protects against much of the spam sent your way, no filter can catch every junk message. (For more aggressive anti-spam application options, look here.)