As Horde/Imp is PHP based, there are a few places that need to be adjusted:

  • /etc/php.ini: memory_limit = 20M ; needs to be 2-3 x max upload/attach
  • /etc/php.ini: file_uploads = On
  • /etc/php.ini: upload_tmp_dir = /tmp ; changing that will confuse horde
  • /etc/php.ini: upload_max_filesize = 20M ; max inividual attachment
  • /etc/httpd/conf.d/php.conf: LimitRequestBody 536870912 # max individual upload

Reember that IMP works better with safe_mode Off.