SMTP error 451 maximum recipiants breached

[#1]

I tried to forward an email but got this message.  I then tried sending a message to 1 person and got same message.  I have 67 emails in my in box, 39 in my sent box and none in any others.  Cant send emails, please what am I supposed to do?
Please keep explanations simple. Thanks

[#2]

Can you try logging in to your router, press disconnect and then connect. Try sending the email again and it should go through.
Could you also confirm that this issue is via webmail.plus.net ?

[#3]

How do I log in to my router?  Yes I use PlusNet for my emails

[#4]

Hi again,
You can log in to your router by browsing to 192.168.1.254 from a web browser, the username and password should be 'admin' and the serial number for your router