Twice in the same week, on two different computers, I fell victim to the same egregious Mail.app memory leak. Within seconds or minutes of launching Mail, it would eat up all the available system memory, and bring the system to a grinding halt; only a hard restart would work after that. The good news, though, is that there’s a relatively straightforward workaround.
TL;DR: An Apple support forum post has the right solution, which is to force the computer to rebuild Mail’s container. Practically, this will mean deleting all the saved mail and settings, then redownloading or reimporting all messages.
In my experience, though, you don’t have to delete and re-add all your mail accounts (Step 3 in the solution); removing the saved messages and settings did the trick for me.