Then, the "special" mailings begin to arrive. With restraint this is typically a solo here, a special announcement there. This is still mailing list manageable in the eyes of your list. With luck your resource continues to maintain it's value. However, push the envelope with more than the occasional extra weekly mailing or two, and the slow slide begins. No one has time to open that many mailings.
Physically it is not possible, and it goes without saying that mentally no one is prepared to cope with them all... it's just common sense. So, here's the "How-To" to help yourself and your readers. Make GOOD use of your autoresponders! Your mailing list first signup box or mailto link would sign the new subscriber up to ONLY your ezine or newsletter (or any customer mailing list).
Then, because autoresponders are so good at this, on the Thank You page, you just add a SECOND signup box! This second box, using a 2nd autoresponder, could say something like: "In addition to our ezine, would you like to mailing list receive special standalone advertisements? These solos are purchased by readers just like you about new and existing programs.