Delivery Limits

The Delivery Limits page groups together three unrelated but useful safeguards for how mail flows into, out of, and through the SmarterMail server: a list of domains that automated forwarding rules are not allowed to target, a way to give specific senders' messages priority during busy delivery periods, and a list of domain names that are blocked from ever being added as a hosted domain on this server. The page is organized into three tabs: Do Not Forward, Sender Priority, and Reserved Domains.

Do Not Forward

The Do Not Forward list blocks specific domains from being used as a destination for a user's automated mail forwarding rule — it does not do anything to messages arriving from those domains. This distinction matters because of how strict some large mailbox providers are about forwarded spam: AOL and Comcast, for example, do not differentiate between the server that originally sent a spam message and a server that merely relayed a copy of it via auto-forward. If a SmarterMail user's mailbox is receiving spam (or is compromised and generating it) and that user has automated forwarding turned on, every one of those spam messages gets silently relayed through this server to the forwarding destination — and it is the forwarding server, not the original spammer, that ends up blacklisted. Because it is impossible to guarantee that none of a user's incoming mail is spam, administrators sending to notoriously strict destinations often choose to block automated forwarding to those domains entirely rather than risk this server's reputation.

As a worked example: if aol.com is added to the Do Not Forward list, a user on this server cannot configure their account to automatically forward all incoming mail to someone@aol.com — the forwarding rule itself is rejected when they try to save it. Mail arriving from an aol.com sender is completely unaffected, since the list only governs forwarding destinations, not senders. The user can still manually forward an individual message to an aol.com address at any time. The reason manual forwarding is left alone is that it is a deliberate, one-off action taken by a human being: the user is choosing to send that one specific message onward. Automated forwarding, by contrast, silently relays every message that lands in the mailbox — including any spam that gets past filtering — with no human reviewing each one before it goes out, which is what makes it risky at the strict destination in bulk.

To add one or more new Do Not Forward domains, click the New button; multiple domain names can be entered at once, one per line. To Edit an existing entry, click on it and change the domain name. To delete an entry, select it (or multiple entries) and click Delete. When adding or editing an entry, the following option is available:

• Domain Name - The domain that should be blocked as an automated-forwarding destination. Once a domain is on this list, a user who tries to save an automated forwarding rule pointing at that domain sees a notification along the lines of: “Forwarding to someone@aol.com is not allowed.”
  Note: Users can still manually forward individual emails to addresses on that domain. Only automated, rule-based forwarding is blocked.

Sender Priority

Sender Priority lets a system administrator raise or lower how quickly a specific sending address's outbound messages are picked up for delivery relative to everyone else's, when the server is under load. SmarterMail's delivery engine only ever has a limited number of delivery threads available at once (controlled by the Max Delivery Threads setting on the SMTP Out page); whenever there are more outbound messages waiting than there are free threads to send them, the engine fills those open thread slots by pulling the highest-priority pending messages first. In other words, Sender Priority does not change whether a message gets delivered, or skip it ahead of messages already in flight — it changes which messages get selected first out of the backlog the next time a delivery thread frees up. Under normal, non-congested conditions, with plenty of free delivery threads, every message is picked up right away regardless of priority, so the setting only makes a visible difference when the outbound queue is backed up.

As a worked example: suppose a company's marketing team kicks off a 50,000-message newsletter send at the same time a customer emails support with an issue. Both messages land in the outbound spool, but the newsletter batch alone can occupy every available delivery thread. By adding support@example.com to the Sender Priority list and setting its priority to High, any confirmation or reply sent from that address gets pulled to the front of the queue the moment a delivery thread becomes available, instead of waiting behind thousands of newsletter messages that were queued first.

To create a new sender priority override, click the New button. To edit an existing entry, click on it. To delete an entry (or multiple entries), select it and click the Delete button. When adding or editing an entry, the following settings are available:

• Email Address - The specific sending address this override applies to. The address must belong to a user who authenticates when sending (an anonymous or relayed connection does not have an authenticated sender to match against this list).
• Message Delivery Priority - The priority level assigned to this address's outbound messages, chosen from Low, Normal, or High. New overrides default to Normal, which is also the priority every message gets when no override applies, so creating an entry is only useful when it is set to High (to be favored over ordinary traffic) or Low (to be deliberately deprioritized behind it).
• Description - A free-text note describing why the override was created, useful for reminding an administrator (or a colleague) months later why, for example, newsletters@example.com was set to Low.

Reserved Domains

Reserved Domains blocks specific domain names from ever being added as a hosted (locally managed) domain on this SmarterMail installation. When a system administrator creates a new domain, SmarterMail checks the requested domain name against this list; if it matches, domain creation fails immediately with a reserved-name error and the domain is never created. This exists mainly as a safety net against mistakes and abuse: without it, nothing would stop an administrator from fat-fingering, or a malicious actor from deliberately, adding a domain like gmail.com as a locally hosted domain. If that were allowed, anyone could then create mailboxes with addresses ending in @gmail.com directly on this server — addresses that would appear legitimate to recipients but have nothing to do with Google, opening the door to spoofing and phishing under a trusted brand's name.

Two categories of domains are typically worth adding to this list. The first is domains belonging to large, well-known free email services, such as gmail.com or yahoo.com: hosting one of these locally would let any user on this server claim addresses under that domain, which is never legitimate and is a strong indicator of an abusive setup. The second is domains that are formally reserved for documentation and testing purposes rather than real-world use, such as example.com or test.com; these sometimes get entered by accident (for instance, copied from a tutorial or sample configuration) and reserving them prevents a throwaway or placeholder domain from ending up in production.

To add one or more new reserved domains, click the New button; multiple domain names can be entered at once, one per line. To Edit an existing entry, click on it and change the domain name. To delete an entry, select it (or multiple entries) and click Delete. When adding or editing an entry, the following option is available:

• Reserved Domain Name - The domain name to block from being added as a hosted domain on this server. When adding new entries, multiple domain names can be entered at once, each on its own line; when editing an existing entry, only that single domain name can be changed.