Our company offers a hosted application for about 300 merchants. Our application needs to send confirmations to our merchants' customers under a variety of circumstances. Currently, all of our notifications are sent from our SMTP server. For example, if a merchant's customer buys something, we send a confirmation from our email address through our server. BUT...some of our merchants would like the notifications to come from their SMTP servers. Many of our merchants are small and use addresses from Comcast, Gmail, Yahoo, AOL and others for business email. Since all of those services support SMTP, we could of store merchant STMP settings and then use those settings to send the confirmations through the merchant's SMTP server. Note the key assumption: *our server* would connect and authenticate to the merchant's SMTP server using the stored credentials. Our server IP would always be the "source". Question: Is this a bad idea? Suppose we have 20 merchants with Comcast email addresses. If Comcast (or any other ISP) notices that many different accounts are all authenticating and sending email from our IP, will they start to view our IP with suspicion or even blacklist the IP?