As the Org Admin you can choose whether, by default, users receive email notifications when they are sent a project mail.
You can set email notifications to be sent when:
You can also control how much information is included in an email notification. As you can see below, the Entire email body email notification on the left contains the project mail content. The notification-only email on the right does not.
You can set the defaults, but be aware that each user can also change their own notification settings.
The Size limit for notification attachments(MB) field only appears if you choose Entire Mail body with attachments in the Receive notifications via email list.
If you have chosen to include project mail attachments in notification emails, you can limit the total size of all attachments. By default, the limit is 4Mb. If the size of all attachments exceeds the limit, the notification email will contain a link to the project mail, and won't have any attachments.