I had been conversing with someone via email earlier Tuesday, having no problems. However, my would-be last email of the day was returned by the recipient's mail system. The error message was

"SMTP error from remote mail server after initial connection:
host zzzz.zz.zzz [xxx.xxx.xxx.xxx]: 554-zzzz.zz.zzz
554 Your access to this mail system has been rejected due to the sending MTA's poor reputation. If you believe that this failure is in error, please contact the intended recipient via alternate means."

I got this message sending new emails or replying to hers, even a one word email like "test".

I've checked my mail server's IP vs. Spamhaus and a few other blacklists with no black marks. What can I send to her (via personal email) to have her company's IT people check or look for? Adding my email to a whitelist, or my IP, or is there something I need to verify on my end?