The benefit of partnerships (in the business sense) can be huge, however, they can also be a disaster if gotten wrong (but affecting many more people than when personal relationships are not suited).

Today I received an email from a company that provides a service for buying and selling houses telling me all about having laser treatment on my eyes. The permissions I gave were to receive information about related products. I almost wished they had at least tried to justify it, perhaps with how with better vision I would be able to see the houses better, or in the long run I would save so much money by not having to wear contact lenses or glasses I could put it towards my mortgage.  But in a legal and also business sense they should have either gathered the permissions to promote all third parties or only worked with partners that do have a related product or service for instance home insurance or removal companies etc.  This email, even if it does make sense to people who wear glasses, can have a negative impact on the housing brand. 

The above is a clear example of a poor partnership but there are plenty of examples of good partnerships that work very well on email. The online pet community who teamed up with a major cat food and recommended it to their cat owners, the dating company who partnered with a  travel company specialising in holidays for single people. As a business think about non competitive complementary business and cross promote what you are doing. Alternatively talk to list brokers and data houses but make sure the emails been sent out make sense and have some synergy. 

Positive relationships work really well when they can add value to the members or customers of the original brand and the brand advertising can piggy back and benefit from the good relationship the members or customers have already.