I cannot send E-mail to "Safe-Mail" address


New Email
Dear Sir,

Today, I got free mail address of "...@safe-mail.be".
After that, I sent a test mail from my other mail address to my "Safe-Mail" address, "...@safe-mail.be", but I could not send the mail.
I received "Undelivered Mail Returned to Sender" from "Mail Delivery System <MAILER-DAEMON@post3.infonia.net>".
That mail was like below.

"This is the mail system at host post3.infonia.net.
I'm sorry to have to inform you that your message could not
be delivered to one or more recipients. It's attached below.
For further assistance, please send mail to postmaster.
If you do so, please include this problem report. You can
delete your own text from the attached returned message.
The mail system
<...@safe-mail.be>: Host or domain name not found. Name service error for
name=safe-mail type=AAAA: Host not found

Reporting-MTA: dns; post3.infonia.net
X-Postfix-Queue-ID: 49AB84E0C39
X-Postfix-Sender: rfc822; ...@yahoo.co.jp
Arrival-Date: Sat, 13 Aug 2016 14:33:32 +0900 (JST)

Final-Recipient: rfc822; ...@safe-mail.be
Original-Recipient: rfc822;ay250@safe-mail.be
Action: failed
Status: 5.4.4
Diagnostic-Code: X-Postfix; Host or domain name not found. Name service error
for name=safe-mail type=AAAA: Host not found"

What should I do?

I sent another test mail from my "Safe-Mail" address, "...@safe-mail.be", to my other mail address. and could send successfully.

Does this mean I can send mails from my "Safe-Mail" address, but cannot receive mails?

And I tried to ask about this problem to "supportsm@conference.safe-mail.nl".
I tried to send E-mail to "supportsm@conference.safe-mail.nl" two times.
Once, from my "Safe-Mail" address, "...@safe-mail.be" on web mail system, and in another time, from my other mail address.
I could not send E-mail in both time. I receive "error report mail" from "Mail Delivery System <postmaster@safe-mail.nl>" and it says like below.
"----- Error description -----
conference.safe-mail.nl: No MX server found for domain.
----- The message was not delivered to the following addresses -----

What should I do?
I cannot ask a question.

I cannot receive mails in my "Safe-Mail" address?
If so, I cannot use this address. Can I delete my account?

Please answer.