Emails to one particular address are returned

Emails I’ve been sending to one particular friend are mostly being returned as delivery permanently failed.  I’ve tried 5 methods to contact her, always using her same email address:

1.  hitting reply to an email from her - returned;
2.  using her name and email address from my contacts list - returned;
3.  typing her email address in manually - returned;
4.  still using my laptop but contacting via Webmail - success;
5   using my phone with my other email account - success

She doesn’t have a problem with anyone else’s emails.

My assumption is that it’s eM Client that’s the problem.  I’ve been using it for several months and it’s only recently that I’ve had this intermittent difficulty.  It can last for days and then it’s all right for a bit.

All suggestions appreciated -  preferably in simple language!

Can you post the message that you receive containing the error? If you don’t want to post it on this forum, you may send it to me at gary@gmx.se

Thanks Gary - I’ve just passed it to you.

It looks like your mail server, the one that eM Client is connecting to, is blocked by the receiving server that your friend uses.

Sending from webmail or the other account on your laptop works because they are using a different IP address.

For anyone who is interested, Margaret’s MTA is being blocked by AT&T (att.net). 

Thanks so much Gary!  I’ve contacted AT&T as you suggested and am about to contact our server as well, about the IP address that’s being blocked.  Your rapid detective work is greatly appreciated.