This way to monetize the community is very exclusive. The most websites out there do not have such a large number of email notifications sent out instantly. So, why not to offer the space to the companies/advertisers?
To add a banner into notifications emails in your AdminCP go to System -> Site Promotion -> Advertisements and click on the tab Email Advertisements. Click on Create New and fill the form out. Be aware that right now there is no possibility to restrict email advertisements to certain group. Upon saving the form your test banner will be instantly added to all outgoing email notifications! Do not test it on your live environment without restrictions.
Banner size in emails
You will find different suggestions what size the email banner must have. Consider that mails are displayed in different email clients that handle the displayed content in their own way. Generally, it should be safe to use banner with 600px width to be displayed in every common mail client (some sources say up to 650px).
The height of the banner is not really restricted. You can use any height, but be sure, that the banner will be displayed above the notification content. It can be annoying for the user to scroll down for the notification message itself. Something between 100px and 200px should be enough for the height.
How to test email banners
As said above there is no possibility to restrict email advertisement to certain group (like website ads). Once the banner is added it will be instantly inserted in every email notification that is sent through the community. There are still some ways to test it:
- If you have a test installation that is able to send emails. Create a new advertisement. In AdminCP go to System -> Settings -> Email Settings and click on Test settings in the upper right corner. Test email will include your ad banner.
- If you do not have a test installation, create a special test area in your community that is only viewable by administrator. E. g. a forum or database in Pages. While creating an advertisement restrict it to this area in the field Restrict to emails from. Now subscribe to new items (topics or records) in this area. Create another administrator and add a new topic or record in the area with a second administrator. The first one (who subscribed) will get an email including the ad banner.
Generally you should test your banner with as many email clients as possible, ideally on desktop and mobile.