SMTP error, why conformation not send?

-- url of the page with the problem -- : www.marrys.nl
-- HikaShop version -- : 4.4.2
-- Joomla version -- : 3.9.27
-- PHP version -- : 7.3.21
-- Browser(s) name and version -- : firefox
-- Error-message(debug-mod must be tuned on) -- : SMTP fout! Data geweigerd.SMTP serverfout: DATA END command failed Detail: Reject for policy reason SMTP code: 550 Additional SMTP info: 5.7.1

Hello
Since a few weeks I suddenly have that the confirmation is not sent to the customer. The order is nicely finished / confirmed as paid. The administrator will be notified of the payment and of the order. I use the same email address in SMTP and Hikashop. But after the order was first this error:

Note
SMTP error! Data rejected SMTP server error: DATA END command failed Detail: Reject for policy reason SMTP code: 550 Additional SMTP info: 5.7.1
The following from addresses went wrong: This email address is being protected from spambots. You need JavaScript enabled to view it.: MAIL FROM command failed, mx-out.mydomain.xion.oxcs.net Error: too many errors, 421,4.7.0SMTP server error: MAIL FROM command failed Detail: mx-out.mydomain.xion.oxcs.net Error: too many errors SMTP code: 421 Additional SMTP info: 4.7.0SMTP server error: MAIL FROM command failed Detail: mx-out.mydomain.xion.oxcs.net Error: too many errors SMTP code: 421 Additional SMTP info: 4.7.0

Now I only see this error: SMTP error! Data rejected SMTP server error: DATA END command failed Detail: Reject for policy reason SMTP code: 550 Additional SMTP info: 5.7.1

I ask the provider of they can view in the log of the server, but still no answer.
I did find In Hikashop the confirmation. If I send it to the customer again from Hikashop, nothing will happen.

Does anyone have a tip how to solve this?