Mickey talks about appending and why it’s not a good practice.
There are a lot of companies out there who love using append strategies, and who find themselves changing email service providers three or four times a year — always hoping for “a bump in deliverability.” I don’t think that there is any accident to that correlation. They are doing something that results in enough complaints that their reputation is killed and they are forced to move elsewhere to try for a fresh star.
There is no permission associated with appending, so anyone using this as a technique to send mail is sending spam. And, yes, even when you’re doing an opt-in append. Unless the recipient has opted in to have their address in an appending database, that first mail to them is still spam.
Many recipients, including myself, deliberately do not provide senders with email addresses. Going and finding an email address that may or may not belong to that user is bypassing their choice. They opted-out by not opting in. Senders who use append to “fix” that “oversight” are no better than spammers.