What is an e-mail client? Why might I need one? If I am sending e-mail, I log on to my service provider (e.g. Yahoo!) directly. Where does an e-mail client fit in in this scenario?
A client is a program/user that uses resources of another machine, called a server. An email client is a program that uses email resources (reading, sending) from a mail server. Webmail front ends (hotmail, yahoo, web gmail) are all mail clients.
But when you say 'mail clients' you tend to think of stand-alone programs for mail. Some well known stand alone clients: Outlook/Entourage, iMail, Eudora, Thunderbird. Your iPhone/iPod Touch has a mail client. Android phones and Blackberry phones have them. For old school geeks like me, pine, elm, mutt, even mail and mailx.
So, advantages of a standalone client over say, webmail?
One, history, what you're used to. Email predates the web by many years. There simply were no webmail clients because there was no web. The interfaces are cleaner. Very resource light, can run in a terminal, they are so non-featured that there are pretty much no viruses for them.
Another used to be offline access - you could download mail, go offline, read it, compose messages, get online and send. This is less required now with always on connections, and now HTML5/Google Gears allows the tools for offline access in Webmail, though Gmail is the only one I know that uses this.
Another and probably the one most important now is the mail UI. A native standalone-client app has UI advantages over a webapp. I still prefer the tabbing and window usage of a native app such as iMail or Thunderbird than even a very nicely written webmail client such as gmail. There are just some things it can't do. It's even more critical on smaller screen devices such as ipods. A client can use native UI buttons and slides and all where a webapp can't, or not as well.
Downsides to standalone clients:
Requires a download. OK, not so bad.
Requires configuration. You need to know two servers, outgoing (SMTP) and incoming (IMAP or POP3).
Pop3 was made when people had one machine, so the model is you have one client that you always use to get your mail. I check mail on at least 2 machines, 3 if you include my iPod. So getting a consistent view of your mail is very hard. IMAP is a newer protocol, so the model is a view on the server which helps across machines, but not everyone uses it (Yahoo only has POP3 on free accounts for example). I'd never set up a POP3 client for anyone because they'd probably lose track of message locations.
The configuration can be hairy too, with choices of SMTP, IMAP, POP3, and various ways of securing these. Webmail just has a URL, when you open it you log in. All your mail is there, you can go from machine to machine and get the same view. No SMTP ports, no radius authentication if you're unlucky enough to need to care what that is.
TL;DR: a mail client is any program that accesses mail from a server, though it's somewhat implied it's a standalone client. Standalone clients have some advantages (UI mostly), but usually the config problems make it easier to just use webmail clients exclusively.
An email client is the program that lets you send email. The Yahoo web interface is an e-mail client too, but not all services have a web interface, so you can use a desktop client (like Outlook) as a generic client for those services. Or even if they do have a web interface, it might be convenient to have a tool that downloads the email directly to your desktop or smartphone.
Having an internet connection is a given. Nowadays at least. In the past internet was costly. You wanted to make a connection, quickly download all your mails to the mail client application and then disconnect. After reading your mails and writing your own a new connection was made, mails send, and connection closed.
An email client that runs on your computer gives you following advantages over an email client running on the web server of your email provider:
- You can choose the email client of your liking.
- You can use the same email client when you switch providers, or even use multiple providers at the same time with the same client.
- A local email client can integrate well with other software on your computer. For example, you can use a PIM suite to manage your emails, contacts and appointments, interlink them and make your computer remind you when your appointments are coming, automatically send emails to the other attendants, etc.
- You can use digital signatures and email encryption through the use of PGP. The first lets you prove your identity to the recipient while the second lets you send confidential emails. While technically possible with a web email, it's either too inconvenient or too insecure to be feasible.
- Your data is safer. Your email provider might go out of business, or they might decide to cancel your account because you violated their TOS. If you used a local email client and configured it to store all the email, you won't lose your old messages if this happens.
- You have slightly more privacy. While the email provider can still read the contents of all your emails, and know the addresses of all the parties in your conversations, they won't be able to track how often you read email, when you read them, which emails you read, etc. If you delete the emails from the server the provider won't be able to track which emails you've kept. You can also use encryption to protect the email contents as mentioned above.
- If your internet goes down (e.g. there was a power outage, a cable failure, the government decided to shut it down or you just went into some deep cave with no internet connection), you could still access your email if you configured your email client to keep it.
Of course there are disadvantages:
- With a web client you can access your email from any PC without the need to install any additional software.
- You might need to use your notebook at a place where everything but web access is blocked. Then your email client won't work.
- Sometimes the web client offered by the provider is very good and it might provide features that no local client does.
The first two can be remedied if you mix a normal and a web-based client for your email. Some web email services offer you with access through a normal email client, so you can use that to read email when at home or on your laptop, and login through the web interface at other places (although it can be argued that doing so is not secure, and you better do it from a second disposable email account).
Even if you don't really like to use a local client, it might still come in handy when you decide to move from one provider to another. If both offer an IMAP access, you can temporarily use an email client to move your emails from the first provider to the second.