Cannot send mail

[Tony2455]

I'm recently having some prpblems with my Sky emails that I've had for many years. I seem to be receiving them OK but sometimes when I reply to the email I get this message.  "Cannot send mail. The sender address is invalid" 

I can however write and send my own emails, it just seems to happen when I reply to an email. 

I've tried to find an answer but no luck so far. I can however send my own emails, it just seems to happen when I reply to an email. 

[caesarome]

@Tony2455 

Is this happening when you reply to  certain emails or all that you try to reply to ?

[Tony2455]

It  seems to be all of my replies but when I send an email (not a reply) it sends ok.