Create a option to ratelimit outgoing email

[madeITBelgium]

https://www.digitalocean.com/community/questions/restricting-outgoing-smtp-with-exim

http://www.exim.org/exim-html-current/doc/html/spec_html/ch-access_control_lists.html#useratlim

# Log all senders' rates
warn ratelimit = 0 / 1h / strict
     log_message = Sender rate $sender_rate / $sender_rate_period
Slow down fast senders; note the need to truncate $sender_rate
at the decimal point.
warn ratelimit = 100 / 1h / per_rcpt / strict

delay     = ${eval: ${sg{$sender_rate}{[.].*}{}} - 

$sender_rate_limit }s
Keep authenticated users under control
deny authenticated = *

ratelimit = 100 / 1d / strict / $authenticated_id
System-wide rate limit
defer message = Sorry, too busy. Try again later.

ratelimit = 10 / 1s / $primary_hostname
Restrict incoming rate from each host, with a default
set using a macro and special cases looked up in a table.
defer message = Sender rate exceeds $sender_rate_limit 

messages per $sender_rate_period

ratelimit = ${lookup {$sender_host_address} 

cdb {DB/ratelimits.cdb} 

{$value} {RATELIMIT} }

[madeITBelgium]

Added, only add APIs.