[exim] Help with serialize_hosts option

In an attempt to cut concurrent outgoing connections on my server, I set the
serialize_hosts option in my smtp transport. As I understand this, if there
are 100 emails going to one host, only one connection will be made, and the
emails will go out serially. (*if I am wrong here, please let me know)

After setting it, when my system sends a group of 20 messages to one host, a
few go out, then I get the message "connection limit reached for all hosts"
and the rest of the emails to that host appear to just get dropped.

(I have also set the option smtp_accept_max = 20 to keep incoming
connections from getting out of control too).

Just a history. Spammers have been hitting me so hard that concurrent
incoming connections were overloading my server. Then rejections were
overloading me on the outbound side. Some servers were doing the guessing
game, sending me 100 or more emails to different "guessed" addresses on my
domain. The rejections of these were blowing me away.

I am running Exim 4.62.

Any suggestions or criticism will be welcomed, because I am coming to my
wits end.

Thank You,

Terry Shows
Computer Software Specialists LLC
USA
615-382-0600