So whats the best frequency to send out email campaigns ?
It really depends on your industry, your target market but also on how big your company is in terms of customer base. For example if you only have 100 subscribers its tough to keep beating the dead horse. In my opinion the best thing to do is send out an email campaign no more than 4 times per month. Make it interesting with tips and issues. Its no different that a newspaper, if there is a great article people will pick up and read but if its jus the same old they will skip it.
Keep short and sweet. Put yourself in their shoes and send out only stuff that you would also like to read. Again, do not over do it.