Cannot send emails with mail attachment outside.

==== Required information ====
- iRedMail version:
- Store mail accounts in which backend (LDAP/MySQL/PGSQL):
- Linux/BSD distribution name and version:
- Related log if you're reporting an issue:
======== Required information ====
- iRedMail version: 0.8.7
- Store mail accounts in which backend (LDAP/MySQL/PGSQL):MySQL
- Linux/BSD distribution name and version: Ubuntu server 14.04
- Related log if you're reporting an issue: is in attachment
====

Hello,

I'am having the following issue:
I want to send a mail with an attachment of 5MB to another email adress, internal is no problem except from roundcube which stops uploading at 8%. External is for both not working and with imap I get an internal server error.
Do you have any idea to fix this?

Thanks!

Regards Lucas Crijns