“Undelivered mail returned to sender” message

When my customers try to send me an email, they get this “Undelivered mail returned to sender” email, although I do receive the email.

This is the mail system at host relay.mailchannels.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

<forward2office15@gmail.com>: host gmail-smtp-in.l.google.com[74.125.195.27]

    said: 550 5.2.1 [ERN] Recipient is receiving email too quickly. (in reply

    to RCPT TO command)

Is this a problem with my email account, or is a problem with the sender email account?

Thanks in advance