There are ways that you can use to control spamming through Horde web mails :
Few of the configurations are as follow :
You need Horde 3.2/IMP 4.2 or later!
1. Turn on $conf['user']['verify_from_addr'] in Horde 3.2. This will
prevent a user from changing their from address without an email
confirmation - not a hard road block to a throwaway hotmail address,
but another hoop to jump through. Also useful for policing your legit
but perhaps less-than-technical users.
2. Turn on the sentmail logs in IMP 4.2. The driver and the amount of
time logs are kept are configured under "Other Settings" in IMP's
config (or $conf['sentmail']). Once you turn on the driver, there are
two permissions you can set through the Horde permissions API:
'imp:max_recipients' => "Maximum Number of Recipients per Message"
'imp:max_timelimit' => "Maximum Number of Recipients per Time Period"
The first one is a hard limit on the number of recipients for a single
message. The second one is limit on the number of messages that can be
sent in the time period configured in $conf['sentmail']. This is for
preventing someone from sending to exactly the max # of recipients
over and over again.
How to do this :
do it in the horde config file
Administration -> Permissions -> then add sub permissions to IMP
Reference : http://lists.horde.org/archives/horde/Week-of-Mon-20080428/035200.html