Warmup process is a name given to the process to build a reputation for your domain and newly assigned ips your domain is mailing on. 

This process involves sending regular but small campaigns over a period of time & gradually increasing the volume of emails as ISP’s begin to form a positive reputation for the domain and ip address. (Note: if you have been mailing on this domain successfully already at another ESP, then the ramp up will be that much quicker)

During this process you will want to mail your most engaged data. What does that mean exactly? Well engaged means your subscribers that are most active. The ones who are opening your emails and clicking through them. Not the ones who never open and delete your emails. 

Once you have warmed up your domain address successfully on your newly assigned ips, you can send out much higher email volume. (Note: if your engagement is poor then this ramp up will take much longer)

Each mailer has their own ramp up process based on engagement so each ramp up is custom to that mailer. It's not a one size fits all approach. 

It would be great to go out of the gate and send all your mail at once. I hear it all the time. I want to send everything and I want it sent yesterday. However, the truth of the matter is the ISPs and mailbox providers don't know your mailing reputation yet and they will throttle you on the side of caution. As they start to know you a little better they start to allow more to get through. Think of it like dating. You have to put some time in to gain that trust. 

Warming is needed whether you are on shared ips or one dedicated ips. If it was 2006 I may say it was only for dedicated ips but in todays mailing world with the algorithms of the ISP's getting smarter every minute you must build your domain reputation up in order to build a reputation on the ips. 

First, you will want to start with your most engaged subscribers. If you are moving from another ESP you will want to segment out your openers and clickers and replys. These are your most engaged subscribers. 

If you are not coming from another ESP then you are starting from scratch and you will be building a list and this organically will begin your ramp-up.

Next, ensure your subscriber list is not a purchased list or email addresses from people who have not opted in. 

Next, once your list is imported you will want to segment your lists by domain groups. i.e. Gmail, Outlook/Hotmail, Oath, Cable, etc.

Then split your list into smaller Segments. This is easy using the virtual segments.

Make sure that SPF record for your sending domain name is appropriately verified using the check tool.

Make sure your DKIM record for your sending domain is appropriately verified using the check tool. 

Create a customer Image and Click URLs that match your sending domain. This will help with reputation building. 

Make sure your list has been properly cleaned using a list hygiene service. This will remove any hard bounces if you haven't been managing your list properly. This only applies to if you are moving your list over from another ESP. If you are starting from scratch then you will not need to worry about this if you are using a double optin.

The key to warming up is to gradually break in and continue checking engagement. If engagement is low then work on better creative or subject lines and continue. If engagement is high then readjust. 

Lastly, during the ramp-up and even after the ramp up you will want to make sure your complaints are less than .1%