Page 1 of Thunderbird question

For some reason Thunderbird has refused to send an email to an @gmail account from my @sky account though I can send it from my other @gmail account through Firefox.
Is there some issue between Yahoo, Sky and gmail that I don't know about?

Snaps

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My new Flash Fiction blog. All my own work
500ish

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I used to be with it, but then they changed what `it` was.
Now, what I`m with isn`t it, and what`s `it` seems weird and scary

I use Thunderbird, I've not tried that exact scenario, but not seen that happen, your mail client shouldn't care who you're sending from/to, unless you've some rules set up.