Mark Brownlow (who I haven’t linked to nearly enough lately) has insightful commentary on the frequency question.
I really don’t think marketers should be afraid of sending email frequently. There are people who appreciate a lot of email. But I do think marketers should be careful when sending frequently. Good delivery is all about your audience and what you have to offer them.
As Mark says:

If every email you send me makes me $1000, you can send as many as you like. If they’re a waste of my time, then once a month is too many.

The companies with the best delivery bring the best value to their recipients. Sure, there’s the occasional issue where mail goes into bulk or is rejected. But when recipients don’t get the mail they want, the ISPs hear about it. And that costs the ISPs money, so they fix it.
Send mail your recipients want, truly want, and you’ll rarely have delivery problems.

expectations • frequency