If you insist on leaving typos in e-mails, there are three areas, at least, that must be exempt: proper nouns (especially contact names); contact-information numbers (including street addresses); and the URLs of online links. Misspelling someone's name is a guaranteed way to annoy him; so is sending him to the wrong address, brick-and-mortar or electronic.
Online links are particularly tricky because they operate without benefit of human logic. If a link leads to the wrong website, the reader's only option is to return to the original message, manually check the link's URL, and try to figure out where the typo slipped in. Most people won't bother; most others will give the mistake only one chance to be obvious.
If it's important to you that your e-mail recipients follow the links you send them, here are four rules for you to follow.
1. If at all possible, copy the address directly from the page. This leaves no room for error.
2. With a page you last visited some time ago, return to verify it's still up before sending its address. Even if it was right last year, things change quickly online.
3. If no immediate access to the website is available, and the link must be sent immediately, proofread it with extra care. If you spell "California" as "Califronia" and bury that inconspicuously within a long URL, you have only yourself to blame for the results.
4. Be especially careful if linking to a well-known organization. The Internet is full of porn and other unscrupulous sites that mimic the URLs of famous names (e. g., nasa.com vs. the official NASA site, nasa.gov) to lure in careless surfers. Don't aid and abet them.
The Internet is a wonderful thing--but, like all marvels of technology, it must be used according to instructions. Make sure you give people the right instructions.