Is IP 213.120.69.121 [mailomta28-re.btinternet.com] a PlusNet server?

[#1]

I have had an email sent through relay.plus.net rejected by Gmail as my SPF does not include IP 213.120.69.121. This is using my personal domain. My spf record has 

v=spf1 include:_spf-internal.plus.net include:_spf-internal2.plus.net ~all

but Gmail has rejected an email which came from IP 213.120.69.121 [mailomta28-re.btinternet.com] which is not listed in the include lists. Should it? 

I had been using the madasafish include (for some historic reason) which also does not include it. My first thought was that it's a new PlusNet server (being part of BT) and should be included.

But possibly not: I was sending to a btinternet.com address which looks as if it is forwarded to a gmail address, so quite possibly btinternet.com is breaking the SPF, not setting up SRS properly. In which case I can't fix it from my end. Has anyone else seem a similar rejection?

[#2 · Accepted solution]

Updating - my correspondent tells me this is not unusual - so it looks very much like a btinternet failure. I can send directly to his gmail address to walk round this in future.

[#3]

so quite possibly btinternet.com is breaking the SPF, not setting up SRS properly

I'm afraid its quite common for email forwarding systems not to implement SRS and thus fail any SPF checking

[#4]

Apparently Gmail only randomly checks SPF so my emails must have escaped this (for this regular correspondent) up to now. 

[#5]

https://support.google.com/a/answer/174124?hl=en