I've never seen that happen. How are you loading the contact list into your survey? Is it a list that exists in your Contacts or Directory, or by some chance are you creating a New Contact List manually from the Compose Email screen (as shown below)?

Assuming that there's no diagnostic report provided that identifies the issue, I would contact Qualtrics support for help.

https://community.qualtrics.com/XMcommunity/discussion/comment/50517#Comment_50517MatthewM
Yeah, actually I only created a New Contact List manually from the Compose Email. For some reasons that message appears and I'm unable to send the email.

I haven't really used that specific feature so I can't say with much authority why that happened. My suggestion would be to check each column and line you typed in to make sure there weren't invalid characters, errant spaces, etc. present, and that all of the email addresses are valid. If you still don't find anything wrong, I would contact Qualtrics support.

https://community.qualtrics.com/XMcommunity/discussion/comment/50519#Comment_50519Thanks a lot MatthewM I will do that.