Apache James server JMAP HTML to text plain implementation in versions below 3.8.2 and 3.7.6 is subject to unbounded memory consumption that can result in a denial of service. Users are recommended to upgrade to version 3.7.6 and 3.8.2, which fix this issue.
https://lists.apache.org/thread/1fr9hvpsylomwwfr3rv82g84sxszn4kl