Email Service Providers are not completely responsible for a low delivery rate. We usually think that when a mailbox is full, or you receive complaints it is because the list or the newsletter is not clean. Sometimes certain user actions such as, the sender is sending misspelling certain words, or because the recipient felt that they did not subscribe to receive such mailing, that can make a huge difference.
There are other factor that weight as much as the reputation of ESP involving the client sending the newsletter, such as: How clean the list is, and how clean the content of the newsletter is.
How clean the list is:
It is very important to abstain from buying lists, make sure the people you are sending your newsletters are opt-in, it is also recommended that you have your clients update their profile, and preferences, on the type of email they wish to receive. That way you can segment your list even more, and send your clients exclusively what they want to receive.
How clean the information inside the newsletter is:
Always emphasize the fact that your client gave you permission, it is really important to let them know that they gave you permission for you to send them emails; sometimes they will make a complaint against you simply because they do not remember that they once allowed you to send them emails. Make sure that the content you send your clients, is content that they fully wish to receive. Make sure you always send your subscribers, information they have told you on the past, they wish to receive.