If you’re getting the "5.3.4 Messag exceeds fixed limit” error, check on the default vs set values in your /etc/postfix/main.cf using
postconf -d (for the default)
and
postconf -n (for the values you have set manually in the main.cf)
Default for message_size_limit (maximum size per mail) is 10240000 (bytes, ie approx 10MB)

Up this to your desired value, eg 30MB by setting
postconf -e "message_size_limit = 30720000"
specifiying the size in bytes.
You can reset the maximum mailbox size using the same method (below just resets to the default value)
postconf -e "virtual_mailbox_limit = 51200000"

But you knew that already!