Lately I get about 50 % of my confirmation emails back , because they are not delivered. 

I have different messages  I have copied a few  below:

-----Oorspronkelijk bericht-----
From: MAILER-DAEMON (Mail Delivery System)
Sent: Sunday, November 01, 2015 10:31 AM
To: web639
Subject: Undelivered Mail Returned to Sender

This is the mail system at host diana58.plusserver.de.

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

<***>: host mail.heplatform.nl[213.189.3.235] said: 501
    <web639>: sender address must contain a domain (in reply to MAIL FROM
    command)

And another:

-----Oorspronkelijk bericht-----
From: MAILER-DAEMON (Mail Delivery System)
Sent: Tuesday, October 13, 2015 12:46 PM
To: web639
Subject: Undelivered Mail Returned to Sender

This is the mail system at host diana58.plusserver.de.

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

<***>: host mail.zeelandnet.nl[212.115.192.99] said: 504
    5.5.2 <web639>: Sender address rejected: need fully-qualified address (in
    reply to RCPT TO command)

It is important my customers get these mails, because they include payment details.

Is the problem in my website or somewhere else?