Problem with gmail addresses

[100Hours]

I have a new customer who bought items from me outside of the site and asked to be added to my mailing list. She has a gmail address (which, by the way, so do I and is the email used to set up my account) and I am getting the following message when I manually add her details:

"This address is on a domain known for hosting temporary email accounts and cannot be added."

How do I get around this and save this important customer information?

UPDATE: Worked third try. Weird.

[Breffni]

Hey there @100Hours, thanks for flagging this here. 

I had a look around our internal resources and that particular error hasn't come up before. It sounds like the system might have had an issue recognising that particular email address initially. 

Thanks for trying again and letting us know you got there in the end. 

If you do see this error again, please let me know.