Emails blocked by BTinternet as spam

[#1]

This is something I've never encountered before so I'm at a bit of a loss. I've been using Plusnet for about 6 years. I don't use your email service as like a lot of people use Gmail. Main reason is that if I switch ISP I don't have to tell anyone.

I live less than a mile from The Peak District National park & help organise a group of walkers. For the past 6 months or so I send out a weekly email to 43 walkers using Gmail via BCC advising where we're meeting the following week.

This morning having used Gmails' schedule service I got 5 BTinternet rejections with the following message:-

554 Message rejected on 2021/11/12 08:01:06 GMT, policy (3.2.1.1) ID (60C7F4CF20CAD1E2) - Your message looks like SPAM or has been reported as SPAM

So far from the people who have replied, none have received it so they can't even unmark it as spam. The other 38 recipients of the email not using BTinternet have not had any problems. Having done various searches it seems that this is not uncommon. It doesn't explain to me though why now? The addresses I've been using have not changed dramatically & this is the first time I've received a rejection. The email I sent did suggest that we put personal details on a piece of paper say in a rucksack pocket, that in the event of an accident (we experienced an event on our walk this week which was serious)  we could contact a specified individual & information for the emergency services. It asked for people to include telephone numbers etc.

I have resent this to a couple of the rejected accounts but through copy & paste of the original contents with no rejections. Now I suspect that an outfit as big as BT that it will be very hard if not impossible to find out why this happened or how to overcome it in future. I suppose I could send encrypted attachments in future but why should I have to go to these lengths?

Whilst not strictly an issue for Plusnet although they are my ISP, maybe someone in the community or from Plusnet can advise what actions I can take other than advise people to start using something else like Gmail, Outlook. This confirms my distrust of BT as an ISP. I'd rather they just stick to managing the network & forget being an ISP. If I had my way I'd break them up (preferably into tiny little pieces). 

[#2]

Wasn't there a serious problem with Gmail earlier today ?

[#3]

Yes there was but I didn't experience any issues. I generally use Gmail's scheduling service which I did in this case so these mails were sent at 0800 yesterday morning. Out of 43 addresses only the 5 BT addresses were rejected, but I think worse than that BT didn't even deliver to the spam folder of those addresses. Google & other services I use send the mail so that the recipient sees the spam in their spam folder & can make a decision themselves about whether it is really spam.