Mass emailing for links is kind of like walking through a darkened neighborhood throwing rocks at all the houses you pass. I'm not saying you should get hit with a beer bottle or bit by a dog, but if you did that kind of thing on a regular basis, I would not be surprised if such did happen.
email spam a long time ago fell into this unacceptable practices category primarily because there sprouted vigilante groups who organized in a fruitless effort to stop spam, but who take shots at just about anyone that sends one of these activists an unsolicited email.
One of the most likely candidates to file a spam complaint is one of your direct competitors. If you collect sites and emails automatically, and do not review them, you are likely to send some emails to your direct competitors, unless you take extraordinary caution to identify them and exclude them.
GoDaddy has been known to unregister or suspend some domains for sending mass link invitations. Look back through the Forum Library (link is at the top of this page next to "Charter").
There's no substitute for manually reviewing sites. I know of an animal site in a particular State that freely links to other animal sites in that State, and asks for a link from them, but this site specifically states that they will only link to other sites which are about this particular species of animal and in the same State. They further tell you that if you send a link request and you do not qualify, that is, your site is in a different State or is not about that specific species, then they will report you for spam, and they will, too.
Other sites will have more than one email contact posted, one specifically for link exchange, another for sales, and maybe another for customer support. Also, sites that tend to engage in the kind of linking you are seeking, the ones that are serious about it, tend to use online forms for link requests and often state that is the only way they entertain requests. If you send them an email asking for a link, obviously you have not looked at their site very closely. Some may be offended, you might get a spam complaint from some of these sites, too.
You cannot filter sites like that from a spider created database without manually reviewing each and every site, not only for the appropriateness of topic and content, but for the other site's terms and conditions.
I have found that from DBs created by spiders only 1 in 10 sites listed should be contacted by email. 70% of the sites collected will just be inappropriate to request links from, and of the 30% that's left, 2/3rds will prefer online form to email. That means at least 9 out of 10 people that receive your email will not appreciate it, and that will be on your best day. Also, you are likely to get a very small response rate without some personalized email that shows you reviewed the site. Bulk campaigns like this probably have less than 1% success rates. (I really don't know on this, because I've never done mass emails to unreviewed sites.)
There are more reasons not to do this. The main one, which actually supercedes the spam issue, is that you are going to get poor quality links using this method. Why? Because you are not manually reviewing sites. Basically, you'll link to anyone, without discretion, if you send out mass emails to spider-created DBs, and those are the same kinds of sites you'll be getting links from, sites that link without discretion, from sites that are just as desperate for links as you. You end up establishing a footprint that IDs you to a group of sites that cannot be trusted. If you link to a lot of sites that can't be trusted, then you can't be trusted.
So, the reasons not to do mass email link campaigns are:
1. They are not successful (measuring the effect they have on the site in the rankings)
2. They can get your site shut down (having an up site is also a metric of success)
3. It is an abusive and unacceptable practice to many people