Default message_size_limit to 50MB

Add MESSAGE_SIZE_LIMIT variable in .env to allow setting the message
size limit for postfix.

.env

@@ -49,6 +49,10 @@ EXPOSE_ADMIN=no
 # Mail settings
 ###################################
 
+# Message size limit in bytes
+# Default: accept messages up to 50MB
+MESSAGE_SIZE_LIMIT = 50000000
+
 # Networks granted relay permissions, make sure that you include your Docker
 # internal network (default to 172.17.0.0/16)
 RELAYNETS=172.16.0.0/12

postfix/conf/main.cf

@@ -7,6 +7,9 @@ mydomain = {{ DOMAIN }}
 myhostname = {{ HOSTNAME }}
 myorigin = $mydomain
 
+# Message size limit
+message_size_limit = {{ MESSAGE_SIZE_LIMIT }}
+
 # Relayed networks
 mynetworks = 127.0.0.1/32 [::1]/128 {{ RELAYNETS }}