This issue occurs when the body of an email would be larger than the default message body size of 10 KB, which is roughly 5,000 characters. This is a safeguard in place to prevent large jobs taxing server resources.
One way to avoid this would be to provide only the necessary fields in the email alert, and direct users to the datalist itself for additional fields. (example, you may want to exclude fields such as "comments" or other free-text fields which may swell the size of the email body.
If you find that adjusting the alert content does not address most occurrences of this issue, you can also consider adjusting the default size of the maximum body size. Contact support for more info on this approach.