Ah what a tease! It would have been a neat solution.
Being unable to send Mail appears to happen to a lot of people, judging by search results. There seems to be no single and so no easy 'just do this' solution.
Have a look at some of these posts.
Mail will not send - one particular remedy
mail won't send - various suggestions and the odd remedy
where you might glean just the right idiosyncratic adjustment to get your Mail working again.
If still no luck, if you search here, using 'Mail won't send" you will get a lot of hits, but you will need to wade through them as most of the posts are unsolved.
So those links will give you a few things to try. If you find the solution, could you post back what it was.
Message was edited by: roam