Too many distinct from addresses

[#1]

Sometimes when I send email through relay.plus.net it get bounced with

 host 212.159.8.107[212.159.8.107] said: 550
    <212.159.61.36> Too many distinct from addresses. (in reply to MAIL FROM
    command)

 for each of the email addresses I've sent it to.

Is there now a limit on the number of TO addresses you can now use in outgoing email?

[#2]

There's pretty much always been recipient limits applied to outbound mail. That said, the SMTP rejection you've quoted seems to imply the problem is with your sender address, not what's in the 'mail to' demarcation

What client are you sending via, and how are the sender addresses configured?

[#3]

I'm using thunderbird and it only happens when I send email to more than a few people, but then again it doesn't always happen.

Is it even possible to have more than one from address?  The account has two identities associated with it but you can only select one of them at once.

[#4]

I suspect I might have found the problem -

[#5]

What was it?

[#6]

Think it was a compromised computer which was swamping things.

Edited to remove something that wasn't related at all.