We just ran into the same issue (Failed sending data to the peer) and it turned out to be a bad email address (contained two @ signs in the address). I'm sure you've checked for that, but it was easy to miss as we thought we tested different combinations of bad emails.
Thanks @hybird21 and @DavidSta for your suggestions. I don't think it is either of those because I've manually checked the email addresses, which is just a short list of around 11 possibilities (and why would it work on some workflows and not others?) and the workflows that don't work, don't work even when I run them manually from server (so wouldn't be too many parallel requests). I don't have any password details embedded in the tool/workflow.
My next step will be to check the workflows that do work to see if I can identify any differences between those and the ones that don't.
No, could not get to the bottom of this. None of my mails have attachments, other than tables (which they all have - both the ones that send and the ones that don't). I'm still looking for a solution and appreciate your suggestions.
I've resolved mine. The peer it was referring to was the smtp server. It was getting an RFC violation and the smtp was stopping it going out. Check with whoever is admin for your smtp and see if it's that server that's stopping your mails from going out. If they lift some of the restrictions for your alteryx server it may work.