[exim] error 421 on a new exim4 server

Hello everyone,

I'm a newbie and currently my goal is just to set a server with the
default Debian config file and the bare minimum modifications to It
necessary to make It work. I tested my email server and domain with
various web tools, everything seems fine, and my ip and domain aren't
blacklisted anywhere.

My problem now is that I noticed that a specific server,
smtp.tiscali.it, is not reachable from my server on his port 25. It's
reachable on port 465, I've verified that with telnet. I tried to send a
few emails yesterday, and after a while they blocked me. The first
emails I sent on that server weren't signed with dkim, and maybe this is
the reason why they blocked my ip on port 25. Now I see on my logs that
the last email is still frozen, and from time to time my server tries to
send it, but I always receive the error 421.

Could you give me some advice, some documentation that I could use to
fix or better understand this please?

PS

Do you think It's still worth reading the book "The Exim smtp Mail
server" by Philip Hazel?


thanks

Alessandro