I have had several clients say they try to email me and get an error when doing so - they can't get messages to get through to me. What is going on? How can I fix this? I am really concerned other clients have tried to email me and not been able to get through...but I never find out about it.

Below is the message they get (I've changed the email addresses):

From: Mail Delivery System
[mailto:MAILER-DAEMON@gateway31.websitewelcome.com]
Sent: Thursday, November 05, 2015 6:52 PM
To: xxx@xxx.com
Subject: Undelivered Mail Returned to Sender

This is the mail system at host gateway31.websitewelcome.com.

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

<xxx@xxx.com>: host mx-biz.mail.am0.yahoodns.net[98.139.171.245]
said: 421 4.7.0 [TS01] Messages from 192.185.143.46 temporarily deferred
due to user complaints - 4.16.55.1; see
https://help.yahoo.com/kb/postmaster/SLN3434.html (in reply to MAIL FROM
command)