Security - We will get there

I have my servers send out log analysis reports to my web application hosted by a hosting provider. This had been working fine for several years until last month. All my reports were not received after Jan 28. Here is how I get it figured out. It could be the backend database is full. This was my first thought as I haven't backup and clean the db for a while. But it's not the case when I logged into the hosting server and found there was enought space. They must have done something as I am pretty sure nothing changed from my end. So I submitted a ticket to their help desk system. While waiting for the response, I decided to dive deeper for this issue. First I grabbed the smtp traffic and launched my wireshark for data analysis. It looked normal for the smtp protocol but I could not research further after the hello msg and protocol negociation. It's using TLS for the data encryption with hosting server's certificate. So I was left clueless for the captured data streams. Meanwhile, the support desk replied back with some really general error msg: Sender check failure. What does that mean? My server sendmail log also gave me some error message as "dsn=5.0.0, stat=Service unavailable" but no more detail as well. Another test I have done was to send email from other systems to the hosted application, they all went through fine. So looks like there was some change at the hosting provider side but the problem should be fixable just from my end. To be continued :-) And you are welcome to email me your idea, thought or what ever if you want to try this out.