Email address not recognised

I have a contact whose email address is in the form [email protected] but emclient refuses to accept this as a valid address despite it working on my iPad and in other clients

Can you give the error please as without the actual address, or the origin of the error, is is difficult to say what the issue is. A screenshot is best if possible.
How did you enter the address in eM Client? Did you type it manually, or is it synced with the same provider as your iPad? 

Maybe copy it from one of these other clients and paste it into a new message. Look out for leading spaces.