The new Apple touch keyboard has BIU controls at the top right. If you don't see the new shortcut bar, there will be a small tab like line at the top of the keyboard. You can either flick this up (tricky) to reveal the shortcut bar, or dap and hold on the smily (or world) key and turn on Shortcuts.
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You can also select text, then tap on it for the contextual menu, where you will see the BIU controls if applicable. The body of the email is the only place where you will see BIU controls.
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That said, there is no reason why Mail should default to Bold text. You should have to turn it on for each email you create or respond to. It's possible you've enabled an Accessibility feature that makes the text look bold, but it is actually normal and your recipient will see it that way.
Here are the two most likely culprits:
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