Mobile Email Problem

Hi Guys

I have an interesting - to me anyway - problem that I'm trying to get my head round.  Here is the scenario:

A friend is on holiday in Austria and sends me an email, to Thailand, from her hotel, using their free wifi.

I receive the email on my desktop, laptop and Android phone and duly reply to it, from my laptop, using the normal 'reply' method.

The friend never receives my reply and, on return to the UK, finds that the reply has not arrived in her PC inbox either.

Why would this be?

I run Linux systems on the desktop and laptop and my android phone is a Samsung J2 (fairly cheap and cheerful but it does what I need it to do).  All three devices are synchronized to my GMail IMAP account.  My internet service provider is ToT (ADSL).  My phone service provider is AIS - but the data stream is disabled and I only normally send emails via wifi.

The friend has just one PC, running WIN7, and a Samsung J3 phone.  Her IMAP email account is with Plusnet and the phone and PC are synchronized (she says).  Her internet service provider in the UK is O2 - but presumably, being out of their service area means that she was connecting via some other route while away.  She wasn't using 'roaming' from O2 as the SIM data stream was disabled so the connection will have been via wifi.

We are both using Thunderbird as desktop clients, although I don't see any relevance there.

She did some additional testing when she returned to the UK by sending a local friend an email from her phone and asking them to reply to it.  The reply never arrived so it looks as though the problem is not caused by location.  Is there somehow a general problem with replying to emails sent from phones?  I suspect that the problem might go away if she changed her email account to GMail but she doesn't really want to do this because she would need a new email address.

Any thoughts anyone?

Cheers, 

DM