Optimise according to your goals!

One funny thing about email marketing is that 75% of the success of a strategy relies on the Database itself.
You can come up with the finest campaign of all times, if you send it to a poor quality database, you won't get any good results.

This has been said quite a few times already on this blog, I tell it once more: the DB quality is the core of the business.

Now, having that in mind there's a very "funny" thing I've noticed after several years working for an ESP is that each and every database reacts in very different ways according to a couple of elements:
1 - Lead origin :
The way the lead was collected will have a great impact on how the database reacts (in a positive or negative way) to your broadcasts and you probably shouldn't address people the the same way.
People that subscribe to your newsletter on your website are probably expecting a lot more of news from you than some guy that (sometimes "accidentally") opted in to your list through some poor quality coreg campaign on some affiliation campaign.

2 - Your industry :
People also seem to expect a given number of emails per week/month according to the list to which they subscribe.
Some of my clients that run "private shopping clubs" kind of businesses usually broadcast each day to their full DB and get pretty good results and a very low complaint rate. On the other hand, my retailer clients that dare broadcast several times a week (over twice more or less) get an instant increase of the unsubscribe and complaint rates.

Amazingly enough it seems people are likely to react similarly according to these two elements among a database, I can therefore only strongly advise that you try to segment your database according to the origin of the lead and test for yourself what triggers the best results for each target.