When you send an email with a large attachment which you think it complies with the size limits designated inside your postfix configuration, you’ll most probably be still getting “Dec 14 21:35:59 mail postfix/smtpd[55736]: warning: D9105A1BAD1: queue file size limit exceeded” error message inside your server maillog. If this happens, you won’t even get a notification about the failure from Postfix itself, instead you’ll notice a quick Undeliverable message from System Administrator right in your client inbox which tells “552 5.3.4 Error: message file too big“, because Postfix has never accepted your mail.

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