I have spellcheck with both my email client (Thunderbird) and BT Yahoo! webmail (which I rarely use). Assuming you are referring to webmail, in the "compose" box there is a button on the toolbar marked with "abc" and a red tick. That's the spellcheck button; also, misspelt words are automatically underlined in red as I type.
I'm still using Classic Mail, having declined to upgrade to the newest version. I would expect the new one to have at least the same features but can't be sure about that.
Edit: Thunderbird has options to turn specllcheck features on or off, but I can't find any similar options in BT Yahoo! webmail - it's always on.
Where exactly are you having the problem?
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