This suggested that it is the Thunderbird server which is complaining.

There is no Thunderbird server. Neither Thunderbird nor Mozilla are an email provider.

My e-mail address is hotmail.co.uk

Your email provider appears to be Microsoft. You'll need to check with them.

If you believe this is in error please login to your portal

Did you do that?

Thanks for your quick reply. But I don't understand why this alert comes up if I try to send my message through Thunderbird but I don't have any problem despatching it if I send it through the Hotmail portal on my browser. That's why I sought advice from Thunderbird.

After my post I used Hotmail to send the message to two recipients. I had a rejection message from one recipient "avasin06.plus.net rejected your message to the following email address: ………………. Your message couldn't be delivered because the recipient's email server (outside Office 365) suspected that your message was spam. To fix this, try to modify your message, or change how you're sending the message, using the guidance in this article: E-mailing Best Practices for Senders. Then resend your message. If you continue to experience the problem, contact the recipient by some other means (by phone, for example) and ask them to ask their email admin to add your email address or your domain name (the text after the "@" symbol in your email address) to their allowed senders list. " But there was no rejection message for the other recipient.

An experiment showed that the message would go through if the attachment was removed. In the past I have e-mailed many messages with very similar pdf attachments with no problem at either end. I didn't find the article mentioned in the rejection message any help.

I had a rejection message from one recipient "avasin06.plus.net rejected your message to the following email address: ………………. Your message couldn't be delivered because the recipient's email server (outside Office 365) suspected that your message was spam.

So then this looks like a problem with the recipients email provider. There's probably little you can do except following their 'E-mailing Best Practices for Senders'.

Not sure what those guidelines are, but what you can try:

• send the message in plain text instead of HTML
• avoid any HTML signatures

I don't understand why this alert comes up if I try to send my message through Thunderbird but I don't have any problem despatching it if I send it through the Hotmail portal on my browser.

This shouldn't be any surprise. Webmail infrastructure is independent from POP/IMAP/SMTP servers. Therefore, even if webmail works, there can be problems with the SMTP server(s).

Thanks for your comments.

I am still puzzled. I have sent many  similar messages with similar pdf attachments but this is the first occasion when Thunderbird has refused to send the message because it believes it to contain spam.    This is the first time one of these messages has not been accepted by the recipient,  but I have not previously sent this type of message to this particular person before. There have been occasions when messages have not been accepted, but these have always been because the attachments have been very much bigger, this was only 606kB.

On this occasion my experiment suggested that the rejection occurred because either of the presence of the attachment or something of it's content. I frequently correspond with the recipient (but with no attachments) and never have a problem. I guess I am in his addressbook, which should mean I am recognised as not spam. I read through the "Best practices" document but didn't find anything of help.

this is the first occasion when Thunderbird has refused to send the message ...

Thunderbird actually has delivered the message to your email provider's mail server, otherwise you'd have got a different error message.
What happens after that it beyond your (and Thunderbird's control).

... because it believes it to contain spam.

It's the recipients mail server who believes it to contain spam, not Thunderbird.

There have been occasions when messages have not been accepted, but these have always been because the attachments have been very much bigger, this was only 606kB.

You can try FileLink, so that the attachment doesn't travel with the email message. The recipient would then just have to click a link to download the attachment.
https://support.mozilla.org/kb/filelink-large-attachments

I guess I am in his addressbook, which should mean I am recognised as not spam.

That might be true for the recipients email client, but it evidently isn't for the recipients mail server.

I have now found the solution to the problem. To recap, I have a Hotmail address, which is a web based service but use Thunderbird all the time. Thunderbird refused to send 3 e-mails, all with the same pdf attachment, to 7 addresses because it thought the messages contained spam. Going to Hotmail on my browser the same messages all were sent with no problem. The message to just one recipient could not be delivered (because of spam).

  I determined that the problem was with the attachment, which was similar to many I have sent before.  Part of it was an article downloaded from the web which contained links to other web sites. I typed in the web addresses of these sites and deleted to originals with the links.    My message with the revised attachment was sent by Thunderbird with no problem and received by the person whose e-mail client had  rejected my earlier message.
  Thank you christ1 for your useful comments.     I still do not really understand why Hotmail sent my messages when Thunderbird refused.