WORK AROUND & FIX:
You need to change the 'Check spelling' feature.
If, 'Check Spelling: When I Click Send' is currently selected.
You need to change it to either 'As I Type' or 'Never'.
Go To: Mail > Settings > Composing > Check spelling: [change to one of the above]
Confirm one of these new settings are selected, Exit Settings and you'll be good to go.
Apple have confirmed today 8th April 2025 it is a known issue that when sending an email with Spell Check selected as 'When I Click Send' Apple Mail freezes.
Please pass this workaround and fix on to others in other Chats. This issue, since the Sequoia 15.4 [24E248] Upgrade has been driving me nuts!! All good now 👍🏻