How to extend the Outlook maximum email size limit?
This question haunts the web for too many years.
Asking google returns millions of (bad) answers:

In fact, the question itself is wrong.
Because even if you configure Exchange or Exchange online to send very large Outlook emails, like 150 MB, 500 MB, 1 GB or more, yes you can do this (see below), this would be of no use.  

Why?

Simply because such large emails would never reach their recipients; almost no email server accepts emails over a 35 MB size these days.
Your over-sized emails would simply get rejected.

So rather than wondering how to increase Outlook or Exchange or Office 365 limits in terms of maximum email size, you need to ask yourself:
Why do I need to send such large or even gigantic emails? 

The answer is easy:
Because you need to send or share large attached files to / with others.
You and I have never seen even a 1 MB email containing only text, right?
and
Because attaching files to an email (using Outlook or any other email client) is the easiest way to send files to others.

So in fact what you are looking for is a solution allowing you to:

  1. Attach large and even very large files to your Outlook emails (let’s say up to 15 GB), as any other file.
  2. Then send these emails as usual (clicking on the Outlook Send button).
  3. Have some kind of magic uploading your email attached files to OneDrive, sharing them with the email recipients and replacing them by OneDrive shortcuts before you email is finally sent. 

This is exactly what Attach2cloud does (plus other interesting things):

Let’s say you’d like to send these files (18 GB of video files):

Create a MS Outlook email and simply drag and drop these gigantic files to your email. 
No, you WON’T get the usual “The file you’re attaching is bigger than the server allows.” error dialog:

Instead, your dropped files get instantly (i.e. one second later) attached to your Outlook email:

As you can see, Attach2Cloud has added The Attach to Cloud cloud tag for MS Outlook attachments exceeding the Outlook maximum allowed email size limit to the attached file names. So you know that these files will be uploaded to OneDrive when you send your email.
The size of the attached files is only 1 KB. In fact these files have not really been attached to the email. The attached files files you see are placeholders created by Attach2Cloud.

Complete your email and simply click on Send (as usual). Your email is put in the Outlook Outbox (as any other email).
That’s it.
You can continue using Outlook to sort your emails or send other emails.

If you look at your Outlook Outbox, you will see your email being processed. Look at the email subject, Attach2Cloud is uploading the email attached files to OneDrive and displaying the progression of the upload: “Starting to upload 6 attachments to OneDrive…” then “Uploading attachment 1 of 6 to OneDrive” etc.