We set up 'Maximum message size' for Postfix node to 10 MB, and trying to send an email with 9 MB attachment file, but it is not working, why?
This parameter manages the following parameter in Postfix configuration (in command above it is shown in bytes while in Control Panel it is shown in kilobytes):
# postconf | grep message_size_limit message_size_limit = 10240000
According to official Postfix documentation this parameter defines the maximal size in bytes of a message, including envelope information:
Additionally, the e-mail is a text-only protocol therefore all binary part of message (e.g. attachment) is base64-encoded. Base64 is always encodes 3 original bytes into 4 base64-bytes, so the attachment size during e-mail transfer is increasing in 1,25 times.
So in case if it is needed to allow to send an email with 10 Mb attachemnts you should increase the 'Maximum message size' considering the explanations above