what is sc01 and how do I overcome this problem

I have been knocking around email for decades. I have no idea what that code means. But I also have no context in which to place it. A generic google search for the error code related to email indicates it might be a server rejection from the messaging company. See https://support.themessaging.co/hc/en-au/articles/8366116990735-Email-sending-limits I am guessing that one as you mention sending to one person. Note the thing is a daily limit. So you have done your limit for today. Try again tomorrow is perhaps what it should say. But I also got results for Microsoft services (email) https://reviewmyemails.com/emailalmanac/mailbox-provider-updates-and-trends/microsoft-updates-filtering/microsoft-errors-421-rp001-550-sc001 Some company called UK Cheap Hosts https://portal.ukcheaphosts.com/knowledgebase/35/Hotmail-is-bouncing-my-emails-error-550-SC-001.html?language=estonian There is even a website called sco1 guide https://sc01guide.com/ So context is everything.