Rt-mailgate attachements size

I’m having trouble resolving the following problem with rt-mailgate wich cannot pass e few emails with the following error

Jul 8 14:06:06 RT getmail: Delivery error (command rt-mailgate 2693 error (75, HTTP request failed: 413 Request Entity Too Large. Your webserver logs may have more information or there may be a network problem.))

Activating debug logs in apache2 webserver did not give any message related to this error so it looks like the problem is not reaching apache server.

I tried setting the “LimitRequestBody” option in apache to a large value without success.
In my RT_SitConfig, the MaxAttachementSize option is not set.
ExternalStorage is active with :

Type => ‘Disk’,
Path => ‘/opt/rt4/var/attachments’,

emails are retreived via getmail with RT 4.4.1 on a Linux RT 3.16.0-4-amd64 #1 SMP Debian 3.16.36-1+deb8u2 (2016-10-19) x86_64 GNU/Linux

type = MDA_external
path = /opt/rt4/bin/rt-mailgate
user = getmail
arguments = ("–url", “http://rt.company.fr”, “–queue”, “BEG”, “–action”, “correspond”,)

Could you tell me where to look for any revelant logs or options so the messages are delivered to RT ?