Each ESP has its own database of IP spammers.
Primarily the problems arise when you choose to send mail to servers with IP flagged in these databases.
For example, we are only sending standard alerts - registration, password reminder, etc.
Tried different smtp and services.
Was the smtp that "loved" all known ESP. But such polls are very limited in the number of sent messages. In "rush hour" from 17 to 21 GMT 90% return with error like "too many messages".
Was services send mail with a good reputation. All ESP took the mail from him sympathetic. But usually there was one ESP that methyl mail with the IP service as "spam".
So you have to constantly monitor not only the reputation of their IP, as well as the reputation of all IP which incidentally are observed in the service of your letters on the way to ESP.
Ideally, they should not be. But to make a server send mail that will not fall into the SPAM - a great success.
Continuing the proceedings found the following: one of the important factors influencing reputation - % of delivered messages. There should be no less than 95%. If more than 5% of messages getting an error like "address does not exist" - this is one of the hallmarks of spam.
The services like mailgan.com API is checking the correctness of addresses (takes a second). And, to save face, it is important to use this test e-mail addresses before you send.
I will say more - these services can send your emails through the "stained" spam IP and the selected ESP will throw your mail in spam. And the new address You will not receive until you pass the threshold of 5%. As soon as the percentage of "marriage" will be less than 5% (3-4%) - you can contact the server tech support to complain about the fact that your emails fall into spam...
Conclusion: for reliable delivery, you need to use scripts to validate user addresses. Or the reputation of your IP from the ESP may eventually fall.