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Verify your account credentials. No matter what e-mail service you use, most if not all of them use authentication methods such as usernames and passwords. Most e-mail clients, such as Microsoft Outlook, save these details so that you don't need to log in every time you want to access your e-mail, but if your credentials get changed and your client doesn't have the updated information, this could cause an issue with the e-mail address.
If you've forgotten your credentials for your e-mail, most services will also have recovery processes in place in such a situation - this is usually done via corresponding "forgot username" and "forgot password" links located on their website, though it can differ depending on the service provider's resources and security standards. The best thing to do in this case is to contact your e-mail service provider for further assistance.
Ensure that the Webmail service is functioning: Now that we know we have the right e-mail credentials, we want to try and narrow down the issue via process of elimination - most e-mail services, such as Hotmail or Gmail, offer a online "webmail" service which allow access to the e-mail's inbox through the e-mail provider's website via a browser. Try to log in and, if you can access the site, try sending and receiving messages to yourself to see if it functions.
If the problem is not occurring through the webmail: If your e-mail service provider's webmail page loads and works, the issue is going to be either with the e-mail client that you're using, or with the settings in the client itself. Verify that the settings within your e-mail client are correct, according to the information provided by your e-mail service provider - these settings can usually be found in FAQs on their website, or via their technical support options.
If the problem continues to persist through the webmail: If you're unable to load your e-mail service provider's webmail page or the issue persists in webmail, then odds are is that the issue is with the e-mail server itself - this should be reported to your e-mail service provider's technical support immediately so that they can investigate the issue, along with any relevant details (Eg. web browser and e-mail client being used, date and time the problem started, ect...)
IMAP and POP3 are two different protocols, and there are many differences between them. The main difference is that IMAP always syncs with mail server so that any changes you make in your e-mail client, such as Microsoft Outlook or Thunderbird will instantly appear on your webmail inbox. On the other hand, in POP3, your mail client account and mail server are not always synced, which means whatever changes you make to your email account in the mail client will not be transferred to the webmail inbox.
In simple terms, if you are using IMAP and mark a e-mail as read, it gets marked as read in your web-based inbox too, because the changes are happening on the server. However, this won’t be the case if you are using POP3, because the e-mails are downloaded to your device and the changes won’t reflect on the server.
POP3 operates by contacting your email service and then downloading all of your new e-mail messages from it. Once they are downloaded onto your device, they are normally deleted from the email server. This means that after the e-mail is downloaded, it can only be accessed using the same device and if you try to access your email from a different device, the messages that have been previously downloaded won't be available to you. Sent mail is also stored locally on your PC or Mac, not on the email server.
As a much older e-mail protocol, many Internet Service Providers (ISPs) still provide their customers with email accounts that use POP3.
IMAP operates by accessing the e-mail server directly and allowing you to access your e-mail wherever you are, from any device. When you read an email message using IMAP, you aren't actually downloading or storing it on your device; instead, you're reading it from the email server itself. As a result, you can check your email from different devices, anywhere in the world: your phone, a computer, a friend's computer. IMAP only downloads a message when you click on it, and attachments aren't automatically downloaded.
While this is a much newer e-mail protocol, IMAP allows your device to be able to check your messages a lot more quickly than POP3.
There are a lot of things that can go wrong with e-mail services. If you're not experiencing a clear-cut issue of not being able to use the service at all, here's a few tips about some causes of rarer issues.
E-Mail Client Settings: if you're unable to send messages but can receive them fine, or vice versa, then the issue might be specifically with the incoming or outgoing settings of your client. An issue with receiving e-mails will generally indicate an issue with the incoming settings, while an issue with sending e-mails will generally indicate a issue with the outgoing settings.
Other e-mail clients, such as older versions of Microsoft Outlook, may also have a setting to either delete or keep messages on the e-mail server after being read, even in the case of IMAP servers. This may cause e-mails to disappear upon being read, or else cause issues with receiving e-mails from anywhere except that one client.
As a helpful tool, you may click here to access Chasms, a useful website that allows you to simulate different e-mail clients and familiarize yourself with how to navigate the different settings found within them.
Inbox Storage Limit: If you're having issues when receiving messages, but are able to send messages fine, make sure that you know how full your inbox is and, if your e-mail service provider has a quota, that you know your maximum limit. Most e-mail service providers will also provide warnings when your inbox is close to full, so pay attention to incoming messages that may warn such.
Even messages in your trash or spam folders take up space on a e-mail server, so ensure that you're emptying those folders regularly and, if you're missing messages, checking those folders to make sure your message didn't end up hiding in one of them.
If your e-mail service provider is similar to most and offers a web-based inbox or webmail service in addition to the option of using a client program, be sure to check both, as older POP3 services may cause the two accesses to not sync perfectly together.
Bounce-back Messages: If you're unable to send messages, keep a lookout for bounce-back e-mail messages in your inbox that may explain the issue. The message itself, automatically sent by a webmaster address in most situations, will typically provide a reason for why the message could not be sent, such as if the recipient e-mail address no longer exists or does not have enough space in their inbox to receive the message.
Antivirus and Firewall Settings: If you're unable to receive messages, in very rare cases, a e-mail server can conflict with a device’s antivirus or anti-malware software, especially if the device has a very strict firewall. Verify that your firewall isn't blocking any ports that may prevent e-mail messages from being received, and set up proper exception rules for those ports if necessary.