Thunderbird is an open source lightweight email client by Mozilla.
By default it doesn’t do spell check before sending emails.
In order to turn the spell check on :
1. Go to Tools > Options
2. Navigate to Composition tab and select Spelling.
3. Enable the tick next to Check spelling before sending .
( By the way, if you have been using Outlook just for sending and receiving emails and nothing else, do try using Thunderbird. You might be pleasantly surprised by the experience.)