The Problems with Free Email Services
Is "free" really free?
Almost nothing is really free; although you are not paying for free email, you will be indirectly paying in other ways. In general, companies offering free email services are not doing so out of kindness... they expect to gain something in return, either tangible benefits (i.e. cash) or intangible benefits (name recognition, happier customers etc.)
Many free email services are supported by advertising, especially web-based email services. With a web-based service you will generally view one or more banner advertisements every time you open an email message. Some services rotate banners every 30 seconds or every minute to expose you to the maximum number of advertisers.
Other free email services, such as POP mail services, make their living by reselling or reusing your email information. For instance, they might send you one ad message a week, or sell your name to companies in certain fields (some free email services require you to complete a personal profile when you sign up; this is to better target the advertising they offer you).
How long is a "lifetime"?
Free email services often market themselves as being "free for life"... but they should really say "free for the life of this company"! Every week, several free email providers disappear, either through lack of funds, or buried under systematic abuse from spammers.
Try to check how long the company you want to sign up with has been in business; if it's really crucial that your free email service remains available, choose a company that also have a for-fee email service, such as Fastmail.fm or one of the largest services, such as Yahoo Mail. If you're less worried, some of the smaller companies offer more services than the giant companies they are competing with.
A dirty little secret: guilt by association
Here's something that most free email providers will never admit: because their services are often abused by spammers, you may find that some ISPs (Internet Service Providers -- companies providing web access) have taken to automatically blocking mail coming from some free email domains. You may well find that your messages will never get to certain people, no matter how many times you send them out; that's because their ISP is filtering the mail before they get a chance to see it!
This is one of the reasons why free email addresses are fine for individual use, but not a good idea if your business depends on people getting through to you.