550 error when sending an email

I am unable to email an email address I’ve got set up, it bounces with a 550 error. I contacted my hosting provider, they suggested it has something to do with me having my domain on Cloudflare

host elara.servers.prgn.misp.co.uk[91.146.105.202]
said: 550-Please turn on SMTP Authentication in your mail client.
550-mailout-l3b-97.contactoffice.com [212.3.242.97]:55300 is not permitted
to 550 relay through this server without authentication. (in reply to RCPT
TO command)

That is not a Cloudflare setting. And neither of those IP addresses belong to Cloudflare. This has nothing to do with Cloudflare.

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