Don't mix up towels and dishcloths

This French saying is so true when it comes to email marketing !
If you own a database that you use to communicate with your clients and then all of a sudden, you plan to rent it, then it's probably one of the worst ideas ever to broadcast it on the same environment as usual (by environment I mean IPs and broadcasting domain or email mainly).

I recall a presentation I attended in Madrid where the hotmail representative explained that transactional emails shouldn't be broadcasted on the same IPs as the newsletter.
I do not agree 100% on this, I tend to believe that if your optin process is clean and that you treat your subscribers with care, the Newsletter shouldn't jeopardize your transactional emails' deliverability. On the contrary, transactional emails could slightly improve your overall (already good) reputation.

BUT, when it comes to sending information to your clients on one hand and sending advertising on the other: YOU HAVE TO SEPARATE BOTH.

According to measures I made roughly on my client's broadcasts, complaint rates are twice as high.

Advertising for your brand and advertising for a third party company are two different things and as such are percieved differently by your subscribers.

Why take the risk?