Why is the mail application on my MacBook crashing?

Open ‘Console’ (in ’/Applications/Utilities’). In the sidebar, select ‘Console Messages’ under ‘LOG DATABASE QUERIES’ (if there is no sidebar, click ‘Show Log List’ in the toolbar). Then click the ‘Clear Display’ button in the toolbar.

Open mail and get it to do it’s thing. Then in Console, there should be some log entries describing what went wrong technically. They may point to the problem.

(If you post the last few lines after mail crashes it may help us get to the bottom of this..)