Error 421 For Undelivered Emails

I had a few orders come through in the last few days and customers said they did not receive their email confirmation for their order. after 24hr I receive a message with a 421 error from yahoo 

"421 4.7.0 [TSS04] Messages from 23.235.198.70 temporarily deferred due to unexpected volume or user complaints "

it's not all yahoo addresses, just a couple, Stats->Email logs shows they were sent. I tried resending but the customer did not receive the resend either. I also checked the Opt Out list and they are not on there either

Anyone have any ideas? I also contacted inmotion hosting to see if they could help.

This isn't a Sytist issue. See: https://help.yahoo.com/kb/att/SLN35446.html