Replies to emails rejected

[#1]

        When I reply to an email, even to a single email address I get this  message. All I can do is write a new email to the person or persons I wnat to reply to.
    552 XrXT1r0090iFVW701rXVHs 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.
HELP!!

[#2]

What email application are you using, Webmail or a standalone client? Are these replies to a certain individual or organisation, or is it happening to all email replies? The error message suggests that our anti-spam platform doesn't like either the email address you're trying to send to, or something to do with the content of the message.

[#3]

I use Outlook as part of Office 365. I have four email accounts for business and private use. My Microsoft Exchange account is not affected, it is the three POP/SMTP from which I cannot repy ot forward emails. Operating system is Windows 10.
Regards,
Gerard King.

[#4]

Try replying to one of the messages using Webmail. It should fail with the same error message and the email will end up in your drafts folder where I can take a look at it (assuming you don't mind of course?)

[#5]

I do not use a Plusnet email account, how will you be able to see my other email other accounts? I do not mind what you look at.
Plusnet is our ISP.
Gerard.

[#6]

So who are your POP3 accounts with? The error message you've provided suggests that you are using Plusnet's SMTP server (relay.plus.net) to send your messages. If you have third party email accounts with other providers, then you may be better off using their servers to send your email.

[#7]

Quote from: Bob

The error message suggests that our anti-spam platform doesn't like either the email address you're trying to send to, or something to do with the content of the message.

I've had the same oddity when I've tried to return an attachment which my virus checker didn't like to the person/company who sent it. It's always puzzled me that the system is happy to deliver a nasty to me but is a lot less keen for me to send it back. Does it go through a different filter? Aside from anything else it makes it difficult to inform a company that they're sending out dubious attachments.

[#8]

It's the same filtering engine but bear in mind it's dynamic. It's possible for the email not to be considered spam on the way in, and then an update get pushed that causes the system to start seeing the email as spam.
Aside from that, I think there are some slight differences between inbound and outbound config. Inbound can be turned off for example, whereas outbound can't.