My company is sending monthly newsletters to its database of... I don't know.... 20,000 members, and some (I haven't counted how many, but a notable number anyway) of the recipient's emails send us an automated reply stating that the IP we're sending from has been marked as SPAM, and that the recipient can not receive the newsletter.
Now, I checked our IP (I'd prefer not to mention the IP, for security reasons) in various blacklisted IP checkers found on Google (I used 3-4 such checkers) and none of them says that the IP is blacklisted anywhere.
So... what's the issue then? Why don't our newsletters go through. Normally, if there were only several out of 20k newsletters that wouldn't get through we would be OK with it. But each month there are more (like last month I think there were 5-10% of newsletters not going through, and that's a considerable amount of failures to ignore) of such messages coming through, and we don't know what is wrong.
Can someone please explain what's causing this, and also how to fix it (if it's something that we can do from OUR end)?