hopefully someone can help and advice ..We have an issue related incoming mail size, we cannot received email larger than 15 MB. but we able to send an email above 15 MB.
for information we do not setup Proxy SMTP-Incoming on our watchguard M270

and our Mail server we setup 30 MB in and out ( mail size ).

Thanks and Regards


  • There are encoding issues which increase the real size of the SMTP being sent, which is one possible reason. However, this is usually only a 33% increase.

    What is your Email server?
    If Exchange, there are a number of settings which can limit the size of received e-mail.

  • Hi Bruce

    thanks for the response , we using Mdaemon for Mail Server

  • What do your Mdaemon logs show for these large e-mails?

  • Sender will receive Bounce back message " 552 Requested mail action aborted: exceeded storage allocation "
    on no show anything suspicious " DKIM and SPF " is pass

    your statement above is correct " There are encoding issues which increase the real size of the SMTP being sent, which is one possible reason. However, this is usually only a 33% increase."
    i just test send mail from my private mail with attachment 16,8 MB and as usual not receive the email.
    an i see the Logs SMTP(in) Mdaemon server the size of the file
    "Thu 2021-12-16 16:09:29.051: [415687] <-- MAIL FROM:<> SIZE=23594801
    Thu 2021-12-16 16:09:29.051: [415687] Performing IP lookup (

    my question , if we not setup of SMTP-in proxy action on firewall policy, that action will use by default of the appliance M270 ?
    i saw in general setting it setup 20 MB for mail size

  • I'm not sure exactly what you are saying about using a SMTP proxy here.

    If you do not have an incoming SMTP proxy in your config, then your firewall should not be limiting anything related to SMTP.

    If you are seeing in your Mdaemon server logs that it is sending a 552 to the sending SMTP server, then your Mdaemon is the cause of these.

  • If you are using a packet filter policy to direct mail to your server then the firebox won't restrict mail size as there is no setting in a packet filter to do so.

    Are you utilizing a mail gateway through your ISP or other third party to direct mail to your firebox? Email size can be configured through there. If not, I would look at your mail server as Bruce says.

    It's usually something simple.

  • james.carsonjames.carson Moderator, WatchGuard Representative

    Hi @Resindo

    You can change the max size in your proxy settings. However, most mail servers /MTAs have a max size of 25MB. You'll need to change the max size on every server in the chain in order to get larger email through. The way messages are encoded also inflates the size of the files attached, so you'll need some overhead.

    If you're using a packet filter, the firewall won't do anything related to size checking, and simply pass the traffic. The size setting will likely be on whatever server rejected the message (You'll usually get a nondelivery report (NDR) from the last server to successfully get the message saying what server rejected it.

    -James Carson
    WatchGuard Customer Support

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