message rejected due to spam or virus when sending email

[#1]

Trying to send an email and I get this message pop up...
An error occurred while sending mail. The mail server responded:  lvWj1i0033uZ86k01vWkhE message rejected due to spam or virus. If you believe this is in error please login to your portal or contact your ISP support team.. Please check the message and try again.
Server settings are:
relay.plus.net
port 25
username (main plusnet name)
Authentication method Password transmitted insecurely
connection security none
Mail client = Thunderbird
What next?

[#2]

The plusnet SMTP server does not like something in your email - it appears plusnet now scan outgoing mail as well as incoming. Were yopu eplyng to an email that was marked spam? if so and you have replied with the original email & attachments intact then you won't be able to send it as it is.
Unless you can change the contents (maybe zip/rename any attachments) there is not much you or plusnet can do. Not sure is disabling plusnets A/V will also stop outgoing mail scanning but may be worth a try - however, if plusnets systems think it is spam the receivers end may do also and it may get rejected there as well.

[#3]

It was a brand new, typed by me, plain text email.
There was a link to a website in it.
When I removed the website link, Plusnet was happy to send the email.
Note: The website link was not a spam site or anything dodgy.

[#4]

Changing your spam settings will not make a scrap of difference - they apply to incoming mail only.
This new spam system is an absolute PITA!

[#5]

Me too.  
Because I've been receiving scam sms on my mobile I've been asked to send the details to ico.gsi.gov.uk.
However - my email contained only a mobile number and the short message about PPI compensation I received. Nothing dubious here. Not even a link.
Attempting to send via PlusNet gives this message -
"qzDo1i00204VyZJ01zDriQ message rejected due to spam or virus.
If you believe this is in error please login to your portal or contact your ISP support team."
I changed to my own mail server and it went ok.