If you are thinking of building a free email service from scratch, then you may want to consider the ideas described in this article about the perfect free email service

If you want to go with an existing free email service provider, the companies below offer a wide range of possibilities to suit every site and budget...

BigMailbox
This free service enables you to offer free web-based email to your site visitors of the form joe@j0e.us (you can specify the domain). You can also show banner ads to users, and even mail all users in one operation to share news, information about new features and so on. In return, BigMailbox's parent company GoHip takes 80% of all banner impressions on the free email site, and also inserts small branding messages into the site and email messages.
User Comments (1 positive, 0 negative):-

March 11, 1999: "Currently I have not had a problem with BigMailbox. Their turnaround said 5 days, it was about 6 or 7, I can understand a couple of days. When they submitted my information to me, their site was easy to manage and quick to respond. When I did run across a problem they responded without delay. I highly recommend them as an email provider because they seem to be the only company on the ball."

BN3
BN3 offers a simple way to set up your own domain name-based free email service. It offers both web-based email and POP3/SMTP email for greater flexibility. Each email account comes with 2MB of disk storage for saving messages. The site itself is very simplistic and some of the links appear not to work, but I have seen several sites powered by BN3 and they can be made to look quite attractive. 

CanBox
CanBox powers the free email offerings of Computer Currents and BroadcastMusic.com, among others. They offer unified messaging services that can include email, voice-mail, SMS connectivity, fax and more. CanBox provides their services in English, German, French & Spanish. Contact them for rates & specifics of their services. 

Chek
Chek powers the free email offerings of a large number of sites, including some very well-known online properties (recent examples include Budweiser and PGA.com). Most sites are template-based with additional customization. Chek offers web-based email and checking of existing POP3 and IMAP-based email accounts.

CommTouch
CommTouch provides the free email services for many large organizations, including regional Excite offerings, NTT, FortuneCity and many more. Many different levels of service and pricing are available.

Coconut Software
You may be more familiar with this company's product name, FetchMail. Coconut Software offers a wide range of services relating to own-brand email. You can choose from various server-based products, where you own the user base and control all advertising on your email site. Alternatively, there are various co-branding, revenue-sharing options you can also consider. Some of the features of their flagship CWPro server software include unlimited nested folders, message filtering, attachments, POP mail and mail forwarding in addition to web-based mail, and much more!  You can run the software on your own server, or have colocation arranged by Coconut Software.

EMUmail
This company offers web-based email server software that can be purchased and downloaded on a local server. This provides web-based email with a variety of features, including address books, folders, spell checking, POP mailbox access and more. EMUmail can also be outsourced under two pricing schemes: a cheap scheme in which EMUmail adds advertising and shares part of the profits and a more expensive service provided on a per-user basis, but without EMUmail advertising.

Everyone.Net
Free email takes another big step with Everyone.Net's free email partnership service! Now you can provide branded email from your own domain name, totally free of charge. You can brand the email service in a variety of ways via the control panel you are provided for managing the service. You get to split the advertising space (banner on site, text message in each email message) 50/50 with Everyone.Net, but you have to sell your share of the ad space yourself. Note that Everyone.net retains all rights to the user base and the user registration information it collects.

I-P
You can set up an instant free email service for your site (you can offer your visitors email addresses of the form address.i-p.com) and there's no cost to join up. You can customize various aspects of the presentation of "your" free email service. You co-own the database of users, and can split the advertising revenue with Outblaze (make sure you read the small print for the full details on how this works). I-P mail offers a wealth of features, including auto-reply, multiple signatures, external mail collection, address books, time settings, filters, anti-spam measures and much more!

Mailworks
Mailworks offers cobranded web-based email solutions (and remote POP mailbox checkers) with various revenue-sharing options.

MightyMail
MightyMail (now owned by Xoom) offers a top-end package underpinned by an Oracle 8 database and supports rich mail formats and graphically enhanced messaging with a choice of letterheads.

Outblaze
Outblaze offers private label portal services. Outblaze will help you set up your own free email service under the domain name of your choice, and the look of the site is highly customizable. In return, you agree to split ad revenues from that free email site 50/50 with Outblaze. The most frequent arrangement is for web-based email, but IMAP support can also be provided upon request.
User Comments (1 positive, 1 negative):-

April 16, 1999: "leonlai.net is a service powered by Outblaze, and I run it. While I cannot vouch for the performance of other web sites, the powered by Outblaze service has been running excellently for me. As far as the operators go, my agreement stipulates that customer support is managed by myself, which means that if the operators do not respond then that is a problem of the provider of this service, and not necessarily a reflection of Outblaze in itself."

March 11, 1999: "Outblaze is very inefficient. They have the sign-up form and all, but when you sign up, you never get a response back. I have signed up numerous times from way back in January, and they still have not contacted me."

PinMail
PinMail offers various options for providing free email services to your site's visitors. The basic free service lets you distribute email addresses of the form address.pinmail.com where "address" is whatever you specify. There are various paid options that will allow you to tailor PinMail to your site's look and feel.

SiteMail
If your site receives over 1 million page-views per month, talk to SiteMail! They may be willing to set you up completely free as a provider of web-based email under your own company's branding.

X and Mail
For organizations seeking to set up their own free email service, X and Mail offers a variety of web-based email solutions from around $10k.

ZapZone Network
@yoursubdomain.zzn.com
This powerful service from CommTouch enables ANY site to provide its own co-branded email service free of charge. Just choose your subdomain and you will be able to offer visitors "@yoursubdomain.zzn.com" web-based email addresses completely free of charge. Note that this service will show CommTouch-sourced adverts to visitors using "your" mail service. Reports indicate ZZN does not work with WebTV.
User Comments (3 positive, 1 negative):-

April 10, 1999: "Currently the presentation needs a bit of work, but that is not a large problem. Signup is fast and the only other problem I have had is the speed of their service - I have had to remove them from my site because it was far too slow."

April 10, 1999: "I wanted to comment on the ZapZoneNetwork. It's totally awesome! You can pick out your own domain name for free! I have about 5-10 domain names from there. It's really cool! And you can customize it so you can put a certain type of wallpaper that goes along with it and change the color of the font and it's really great. I've never had a problem with it."

April 2, 1999: "I have found that ZZN has been very reliable and has driven more traffic to my sites. With the "Mailing List" feature, I can send out announcements about my website to all of the users. It has met my needs and helped me a lot with my website!"

March 7, 1999: "Using Zap Zone Network, I have created free email services for 2 web sites I maintain. This is one of the best freebies on the web for webmasters (even amateurs like me). The service is customizable and seems to be reliable. One warning: like many web-based email services, access can be flaky from behind a firewall."