Since forwarding emails to outlook from gmail, I’m having trouble sending mail

I've forwarded all my mail so that I don't use gmail but since then emails from a specific company has capital letters after the "@" and I'm getting messages that my account is having trouble sending messages to them. If the messages don't fail instantly then they will go to my outbox without informing me which is incredibly inconvenient because urgent mail doesn't get sent. What can I do?