When a forwarding rule, alias, or filter sends mail back to the same address that originally received it, the mail system creates a delivery loop. After the message is passed around enough times, the mail server stops trying and returns a non-delivery report with a too many hops error.

What “too many hops” means

A “hop” is one delivery step from one mail handling stage to another. For example:

• the message arrives for user@example.com
• an alias or filter forwards it to user@example.com again
• the server treats that as a new delivery attempt
• the same rule runs again
• the cycle repeats

To protect the server from an endless loop, the mail transport agent counts these re-deliveries. Once the count exceeds its safety limit, it rejects the message and generates a bounce such as:

<code class="language-text">554 5.4.6 Too many hops
</code>

or similar wording depending on the MTA.

Why this happens in Virtualmin/Usermin

Virtualmin and Usermin allow several kinds of mail handling rules, including:

• mailbox forwarding
• aliases
• server-side filters
• autoresponders in some cases

These are only interfaces for the underlying mail system. If a rule is configured to deliver to the same recipient again, the underlying MTA still sees that as a loop.

Common examples include:

• user@example.com forwarding to user@example.com
• an alias for sales@example.com that points to sales@example.com
• a filter on info@example.com that redirects matching mail to info@example.com
• two addresses forwarding to each other, such as:
• a@example.com forwards to b@example.com
• b@example.com forwards to a@example.com

Why “keep a copy and forward” is different

Forwarding to an external or different internal address is normally fine.

For example:

• john@example.com forwards to manager@example.com
• sales@example.com forwards to crm@externaldomain.co.uk

These are valid because the destination is not the same looping recipient.

If the goal is to retain the message in the original mailbox as well, the correct action is usually:

• deliver locally, and
• send a copy elsewhere

It must not re-inject the message to the same local recipient.

How to avoid the problem

Safe configurations

Use one of these approaches:

• deliver mail to the local mailbox only
• forward mail to a different address
• forward mail to multiple different addresses
• use filters to move mail into folders rather than redirecting it back to the same address
• use aliases that expand to real destination mailboxes, not themselves

What the server is protecting against

Mail systems enforce hop and loop detection because a self-referential rule can otherwise cause:

• endless queue growth
• high CPU and disk use
• duplicate messages
• backscatter and unnecessary bounce traffic
• general mail delivery disruption

The too many hops error is therefore a safety mechanism, not a fault in Virtualmin or Usermin themselves.

Summary

A too many hops NDR occurs because forwarding, alias, or filter rules that point back to the original recipient create a mail loop. Virtualmin and Usermin allow the rule to be defined, but the underlying mail server detects that the message is being re-delivered repeatedly and eventually stops it.

The rule of thumb is simple:

• forward to a different destination
• deliver locally if a copy is needed
• never redirect or alias an address to itself
• avoid circular forwarding chains between local addresses