Thanks for the info, I have a clearer picture now. I did look at the Fastmail help pages but also wondered why it was working now as I had never set up an app password for her. When I originally set it up, the iOS devices just auto configured the servers but I had to change the outgoing servers as it was only a guest account. That's also why I didn't use the Fastmail app originally. She's used to the iOS mail app now. Anyway, she's now gone on holiday without me having made any changes to the SMTP server so no doubt she will wonder why she can't email when she has paid. I did explain to her but some people you just can't reach.
I'll have to sort it when she gets back.