SendGrid keeps failing

Each time I try to send emails via the SendGrid app it keeps failing.
It has worked successfully previously, but now it doesn’t. I don’t think I’ve changed anything, and I’m only trying to send to 40 email addresses.
My email is fairly short, only 330 characters (only 50 words). I’ve tried using a new API key but that doesn’t work either.
Does anyone have any ideas on why this could be?
Thanks in advance.

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