Zimbra with maxmessagesize

The message_size_limit postconf parameter is configured globally for all Zimbra MTAs. The default Zimbra MTA configuration uses the default Postfix message_size_limit of 10MB (i.e. '10240000' bytes ~10MB). Note that this is the size of the full RFC 2822 internet message, after any necessary MIME-encoding.

An Initial Warning On These Values

Please do not set these limits too high, as postfix will check to confirm you have the free space on the partition. For zimbraMtaMaxMessageSize, it multiplies the variable by 1.5 and if the free space isn't available on the partition it will not send ANY messages - effectively causing an mta outage. You'll see a message in /var/log/zimbra.log like this if you set zimbraMtaMaxMessageSize to high :

Jan 15 11:18:53 SERVERNAME postfix/smtpd[29088]: NOQUEUE: reject: MAIL from smtp.DOMAIN.com[70.43.218.70]: 
452 4.3.1 Insufficient system storage; proto=ESMTP helo=<mail.DOMAIN.com>
Jan 15 11:18:53 SERVERNAME postfix/smtpd[29088]: warning: not enough free space in mail queue: 
161044520960 bytes < 1.5*message size limit

How can I increase message size 20M ?

Postfix configuration (zimbraMtaMaxMessageSize and message_size_limit)

You can examine the current value of this parameter like this:

# su - zimbra
$ postconf message_size_limit
message_size_limit = 10240000

 #su zimbra

zmprov modifyConfig zimbraMtaMaxMessageSize 25165824

zmprov modifyConfig zimbraFileUploadMaxSize 25165824

zmprov modifyConfig zimbraMailContentMaxSize 25165824

postfix reload

noted:

- If you using multi servers (store and mta), you need restart all servers

- For send email with 20M you need inbox free 20*1.5 MB

good luck!