My website visitor cannot complete the registration process for a new account. The XOOPS system reports the following :

You are now registered. However, we were unable to send the activation mail to your email account due to an internal error that had occurred on our server. We are sorry for the inconvenience, please send the webmaster an email notifying him/her of the situation.

The registration process works fine before. Does anyone know what could be the problem ?

it seems a problem in your SMTP.
could you send an email from your site?
check the admin--> preferences --> mail setup
so its better to contact your host for this
regards

You have to configured first and setup properly your mail server. Contact your Web/Domain/Network Administrator for this. I encountered the same problem before but now I already fixed it.