SMTP error 5.7.16 Message is too big for the specified priority

What SMTP error 5.7.16 means

A specialist code from the SMTP priority extension, and one almost no UK business will ever see. The receiving server supports message priority levels and will not accept a message this large at the priority requested — a smaller message at the same priority, or the same message at a higher one, might be accepted. It arrives with 552, and it has nothing whatever to do with the high importance flag in Outlook.

Security or policy status
Sent by A receiving server implementing the SMTP priority extension
Rare

What SMTP error 5.7.16 Message is too big for the specified priority actually means

RFC 6710 defines an optional extension letting a sender attach a numeric priority to a message and a server make handling decisions from it. Servers running in a constrained mode may accept only high priority traffic, and may additionally cap how large a message at a given priority may be. This code reports that second rule. It appears only where both ends implement an extension that ordinary business mail platforms do not advertise, which is why it belongs to military, emergency services and certain regulated messaging environments. If you have found it in a normal commercial bounce, read it again, because a plain size rejection is far more likely.

The message is too big for the specified priority. This condition might be temporary, for example the server is operating in a mode where only higher priority messages below certain size are accepted for transfer and delivery.
— RFC 6710

How 5.7.16 appears in a bounce

A server reporting this condition sends it alongside a three-digit reply code, and a bounce prints the two together. The registry lists 552 5.7.16 as the pairing for this status.

The leading 5 is the server's verdict rather than part of the code's identity: it marks this as a permanent failure, which means the sending server has given up and returned the message.

What causes SMTP error 5.7.16 on a business phone system

A receiving server operating in a restricted modeDuring congestion or an incident, a priority-aware server may accept only small messages at lower priorities while keeping capacity free for urgent traffic. The specification anticipates this being temporary.
A size ceiling attached to a particular priority bandWhere an organisation runs graded priority levels, each band can carry its own maximum size. A message correctly labelled but too large for its band is refused rather than downgraded.
A sending system requesting a priority it should notA gateway or application configured to stamp everything with the same priority level will eventually send something too big for it. The fault lies in the blanket configuration rather than in any individual message.
A gateway translating between different priority schemesBridging a specialist messaging platform to standard SMTP means mapping one set of priority values onto another. A mapping that lands messages in a band with a tight size limit produces this rejection consistently.

How to fix SMTP error 5.7.16

Confirm it is really a priority rejectionRead the full text of the bounce. Standard commercial platforms do not implement this extension, so on Microsoft 365 or Google Workspace an ordinary size rejection is overwhelmingly more likely and should be investigated as such.
Reduce the size of the messageThe condition is about size at a given priority, so a smaller message usually passes without changing the priority at all. Remove or link to attachments and resubmit before trying anything more complicated.
Check what priority your system is requesting and whyIf an application stamps a fixed priority on everything it sends, that is the thing to correct. Set priority deliberately per message type rather than applying one value across the board.
Ask the receiving organisation for their priority and size policyAnybody running this extension has documented rules about which bands exist and what each will carry. Get them in writing and configure to them rather than working the limits out one rejection at a time.

Fixing the underlying problem

This page explains the code. These guides walk through the fix in detail.

Questions about SMTP error 5.7.16

Is this the same as marking an email as high importance in Outlook?

No, and conflating the two sends people down the wrong path. Outlook importance is a header the recipient's client displays and no server acts on. The priority here is a transport-level value negotiated between servers supporting a specific extension, and it changes how the message is handled in the queue.

We are a normal UK business on Microsoft 365. Could we ever see this?

Realistically, no. Neither Exchange Online nor Google Workspace advertises the priority extension, so the negotiation this code depends on never happens. If it appears in your logs, look first at whether a specialist gateway or a partner running a regulated messaging platform sits in the path.

Will it succeed if I simply send it again?

Possibly, if the receiving server was temporarily restricted when it refused you, and the specification allows for that. But this is a permanent class rejection, so the message has already been returned, and reducing its size before resending gives you a far better chance than repeating it unchanged.

Related SMTP status codes

Source. SMTP error 5.7.16 Message is too big for the specified priority is defined in RFC 6710 and registered in the IANA SMTP Enhanced Status Codes registry. The causes and fixes above are drawn from our own experience supporting UK business email systems.

All Email delivery & SMTP codes