Email keeps being bounced back, fails to deliver?

I've never had this problem before. I'm using Gmail. I've tried sending an submission email off to this literary agency, but twice it's been sent back, giving me the message that it's failed to send because of the servers their end. I'm not really sure what to do. It's important I get... show more I've never had this problem before. I'm using Gmail. I've tried sending an submission email off to this literary agency, but twice it's been sent back, giving me the message that it's failed to send because of the servers their end. I'm not really sure what to do. It's important I get this email to them. I've checked the address and it's correct. Why is it doing this?
Update: I didn't have any problems with any of my other submission emails to other agents.
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