SharePoint Online - prevent automatic email notifications

Hi,

We have an on premise installation of SharePoint 2013 for our staff intranet but have access to Office 365 SharePoint Online which we are currently testing before migration. I've set up a test custom list and given some users access by adding them to an SP security group and associating it with the List. When adding users to the security group the 'Send an email invitation' was NOT selected - this worked as expected with the users not receiving an email alert. However a few days later all these users received an automatic email notification inviting them to collaborate on this custom list (see screenshot). Anyone got any ideas how to prevent these emails being sent from no-reply@sharepointonline.com to users in security groups?

Thanks in advance