This is a configuration issue with your email server configuration, see https://community.tableau.com/thread/212624 <https://community.tableau.com/thread/212624> for details. If you are unable to enable your email server to handle larger emails then there are a couple of options that I can think of:
1) Instead of using a single advanced alert that sends one file use multiple alerts that send multiple files, with each adding URL parameter filter to the content reference. For example for data with two geographical regions you might have Region=East and Region=West filters.
2) If you know Python or know someone who does then it adding some code to zip up the CSV file is another option (even better would be to contribute a pull request to the VizAlerts project at https://github.com/tableau/VizAlerts <https://github.com/tableau/VizAlerts>).