A list with 15,000 emails showed roughly 5,000 "bad" addresses. However, when exporting the "bad" addresses into a seperate list and running the 5,000 "bad" emails, it showed that 1,000 of them are "valid". Is there a reason that the first run showed them as bad but the second run showed them as good? I'm guessing that your server tries to connect 3 times per email test and the second run was more successful? Is there a setting that you recommend to avoid this problem in the future?