Cannot send emails from PLusnet account

Since the latest update I have been unable to send emails from my Plusnet account. Plusnet support say that their code is 30 years old so it is possible that it is no longer compatible with modern systems. Following failure message:

[SMTP] Cannot send message (Sending email “Testing” failed with error: Email recipient not accepted by the server.)

sábado 30 diciembre 2023 :: 1333hrs (UTC +0100)

That is strange, we deal with Plusnet in UK, they started less than 30 years ago and are a subsidiary of BT/EE.
I guess from the error you have posted that is from an account setup.
Please state exactly what you are doing and how you are trying to do it.
If possible with screenshots.

¡Buena suerte y Próspero año nuevo!

Alvaro Muñoz (skybat on holiday)

¡Los mejores desde Valencia y mantente a salvo!

[email protected]

Hablo español y catalán, luego portugués, inglés, francés y árabe.
con conocimiento de varios otros idiomas.

Screenshots from account settings:

More to follow