This is a complex question, and must be dealt with on a case-by-case basis. Here are some general tips on importing your list:
Depending on where you live and where your ESP is based, different rules and laws will apply. For example, regardless of where you live, if your ESP is US-based, then the CAN-SPAM act would apply. Your local laws may be even more restrictive, so consider that before choosing an ESP.
Some ESPs, such as Constant Contact, will let you import any type of list as long as you give them your word that the list is kosher, and, unless you have confirmation option turned “on”, you’ll be able to email this list without restrictions. This works beautifully for people who may be transferring an existing list from a different ESP, or are simply planning to start using email to reach their existing clients on a regular basis. With Constant Contact, you can confirm those emails over time, using a so-called “permission reminder.”
However, don’t be tempted to upload one of those “million leads” CDs, because you’ll be labelled as a spammer quicker than you can say, “double opt in!”
If you’re planning to purchase leads from list brokers, most ESPs won’t allow you to import such lists. Notable exception is 1ShoppingCart; they still allow purchased leads from services such as ListBuilder. All imports still need to be confirmed though, unless you hire somebody to manually enter them as “old clients,” one by one, but be very careful with this strategy, if you enter too many “old clients” your account could get suspended, or at least your ability to add “old clients”.
You may be tempted to try to do the list management on your own, by setting up a mail server, and running an Email List software, such as AutoResponse Plus, on your own. This is so complex that I won’t even go there, but suffice is to say that you will have a hard time finding a reputable web hosting company who will allow you to upload purchased lists. Better stick to using an ESP, unless you’re a web-programming guru, and are ready to fight your own battle white-listing your domain with the likes of Yahoo, MSN and AOL – good luck with that!
Once you have imported your list, you will have different results in terms of responsiveness of your list, depending on how “fresh” the list is, among other factors. If you’ve been in business for years and have never contacted your list of clients by email, it is very likely at least some of your email would be labelled as spann, even if you were trying to import a “clean, honest” list of your own clients.
When importing a list expect to loose subscribers, anywhere from 10%, to as much as 90% of subscribers. Don’t despair, though, those who do not confirm their intention to receive email from you are, most likely, not your best prospects anyway, so don’t shed any tears over this.