Is the following correct ?
Thank you, Joe
is it correct with a line space-
I would say the line break is required, as "Thank you, Joe" can read as if you are thanking Joe for something, rather than saying thank you yourself as Joe.
Ultimately it depends on the medium and the context. If it's a formal letter or an email, then put the line space in. If it's something more informal like a Text/SMS or some other shorter medium, then it's unlikely to matter and people reading it will understand the meaning from the context.