I have customer experiencing this issue. Recommendation so far is to increase the JVM size to get around the out of memory error. Not sure how long this workaround can last.
Issues in the knowledge articles are not resolved and it's pointing toward usage and scenarios as the root cause. However, there's no guide or out-of-the-box scenario created to help customer deal with large email. Customer needs guidance on how to create optimised email scenario to cater for large email or have this optimised email scenario available out-of-the-box .
These days, some email include video recording to show steps on how to reproduce a problem. Connect-IT need to cater for such a scenario.