CHECK CURRENT SIZE

[zimbra@mail ~]$ postconf message_size_limit
message_size_limit = 10240000

SET ATTACHMENT SIZE

[zimbra@mail ~]$ zmprov modifyConfig zimbraFileUploadMaxSize 35020000
[zimbra@mail ~]$ zmprov modifyConfig zimbraMtaMaxMessageSize 35020000
[zimbra@mail ~]$ zmprov modifyConfig zimbraMailContentMaxSize 35020000

RELOAD

[zimbra@mail ~]$ postfix reload
/postfix-script: refreshing the Postfix mail system

VERIFY SIZE

[zimbra@mail ~]$ postconf message_size_limit
message_size_limit = 35020000