IP management

Well let's start with the beggining.
When you send out an email the main ISPs will react first (but not exclusively) according to your IP reputation.

A good IP reputaiton is very important. This reputation is calculated according to several items:

1 - The volume of sent emails.
The number of sent email on a daily, weekly, monthly basis must be sufficient. An IP sending a to low amount of email is more likely to be Blocked or Spam filtered.

2 - The number of Spam traps triggered.
Most of the freemail/webmail providers (hotmail, gmail,...) use closed addresses as spam traps. Once a user stops using his email address, it is first inactivated (the ISP then sends back an error to the email sender). After a while, it is transformed into a spam trap. Email senders that haven't updated their database according to the error messages (by deactivating the profile) will then be considered as spammers.

3 - The number of Unknown users.
A high number of unknown users in your database highly increases the risks of being blocked by the ISP and will highly impact your IP reputation. You must make sure your database is clean before shooting it and ALLWAYS unsubscribe hardbounced emails.

4 - The number of spam complaints.
The number of complaints is of course important for your IP reputation (it does impact your sender ID as well). Complaints can be made through the 'this is a spam' button on the webmail interface, to abuse@domain.com email addresses or SPAM software such as Spam cop.
With hotmail, you can negociate a loop that will blacklist users in your database when they click on 'this is a spam' instead of triggering a Spam alert.

To see your IP reputation, you can type it in on the following website:
https://www.senderscore.org/

If you administrate directly your sendouts (without using a send out tool) you can create an account on the hotmail SNDS
https://postmaster.live.com/snds/

If you use a sendout tool such as cheetahmail (US) or cabestan (EU), they should already be registered and can give you the relevant information you might need.