Emails being rejected on send

[#1]

I sent an email to my brother (who is also on Plusnet).

The email contained the following bit of text:

We'd also need to make sure that we get any internally coded urls
changed to HTTPS.

Then its just a matter of changing the server configuration so we run it
on HTTPS but also force any requests to the old HTTP service over to HTTPS.

I could send it to him OK

But when he attempted to reply Plusnet rejected the email.

The only way to get it to send was by either removing my original email or by editing it and putting spaces into the https bits.

It would seem your spam detector is being a bit too paranoid.

[#2]

Chances are it was something beyond this. The content of that message is clean. Does your brother use a signature or something like that which contains additional  URLs?

[#3]

It wasn't anything more than that - he simply put spaces into the http and https bits in my email and it sent it through perfectly.