When an ISP observes email suddenly coming from a new or “cold” IP address, they will take notice and immediately begin evaluating the traffic coming from that IP.  Since ISPs treat email volume as a key determining factor when detecting spam, it is best to begin sending a low to moderate volume, eventually working your way up to larger volumes. This gives the receiving email providers a chance to closely observe your sending habits and record how your recipients engage with your email.

IP warmup initial send volumes need to be spread out over multiple days.  For example, If you plan on sending at most 50,000 emails a week, you should split your lists into several sublists with a starting limit of no more than 4,000 recipients in each list.  Email only one sublist per day over the first few days.  As time passes you increase the daily volume all the while monitoring spam reports, list hygiene, user engagement, domain and IP reputation, content and more.  If you see any issues stop immediately and resolve before sending more.  Otherwise continue until your desired volumes are reached.  

 Consider the following process:

• Start with your best contacts: Identify and collect your very best and latest (under a year old) contacts to start your IP warming initiatives. Using these contacts will enable high delivery rates and will help build your IP’s reputation.
• Warm up slowly: Start sending emails spread out over multiple days. Split up the sending as much as possible.  The slower the better to start with.  Rather than sending out a 20,000 email campaign in one go, split it up to 4,000 emails a day for 5 days. If your bounce rate stays below 10% and your spam complaint rate remains under 0.1%, you can start to increase your volume in the weeks ahead.  Continue ramping up until you reach your desired volume.
• Be consistent: To avoid getting your emails bounced by your recipient ISPs due to spikes in volume, send out a consistent stream of emails over multiple days.
• Increase gradually: Once things are going well, increase your sends progressively over a month’s time. Don’t ever attempt to more than double your email volume in a single month. If you tend to email infrequently, (e.g. only one email campaign per month), avoid releasing more than 100,000 messages per day (although some experts say more than 10,000 is too much).

It is possible to do larger ramp-ups starting with a higher sending volume but you shouldn't do more than 10,000 per day.  Assuming your bounce rate stays below 10% and your spam complaint rate stays below 0.1% on those sends, you can safely double your sending per day over the next few weeks until your intended sending volume is reached.  For example, if you want to send 200k emails a week, you should ramp up like this:

We recommend keeping your sending schedule low to help avoid having your emails throttled by recipient ISPs.  Having a consistent mail volume from one business day to another is better than having a large volume spike on one day of the week and no email sent on remaining days of the week.

You also need to monitor the bounce rates of your lists and stop mailing if the bounce rate of your first one or two sub-lists exceeds 10%. This is a sure sign that your list needs maintenance! Clean up your list, and then resume sending to the remainder of the sub-lists.