Salesforce is making a behind-the-scenes change in Marketing Cloud Engagement marketing teams should not ignore.

On March 16, 2026, Salesforce disables double evaluation in email subject lines. This affects how AMPscript resolves in live emails, journeys, and triggered sends.

If your team relies on dynamic subject lines, this deserves attention now.

You can read Salesforce’s full guidance here:
https://help.salesforce.com/s/articleView?id=000391994&type=1

What Salesforce is changing and why

Marketing Cloud has historically evaluated subject lines twice. This enabled nested AMPscript and personalization inside variables. It also introduced risk.

Salesforce identified a security vulnerability tied to this behavior. As a result, double evaluation is being removed.

After March 15, 2026, subject lines resolve once. No fallback. No silent exceptions.

The pattern Salesforce is targeting

You are likely impacted if your subject line looks like this, or a variation of it:

%%=v(@SubjectLine)=%%

This becomes a problem when the variable itself contains AMPscript or personalization pulled from a data extension, lookup, or script block.

This pattern shows up often in shared templates, legacy journeys, and triggered sends that still do important work.

What Salesforce expects you to do

Salesforce introduced a new AMPscript directive to enforce a single resolution:

%%__SINGLE_RESOLVE%%

You append it to the end of the subject line. In more complex scenarios, the subject line also needs to be wrapped in TreatAsContent().

Common examples:

%%=v(@SubjectLine)=%% %%__SINGLE_RESOLVE%%

%%=TreatAsContent(SubjectLine)=%% %%__SINGLE_RESOLVE%%

Marketing Cloud removes the directive at send time and resolves the subject line once.

What is actually impacted

This change applies only to emails that are:

• Live before March 15, 2026
• Still sending after March 15, 2026
• Used in Journey Builder or triggered sends

It does not affect previously sent emails or one-time sends.

One operational detail matters.

After fixing the subject line, you must manually republish the journey or triggered send. The platform does not apply the update automatically.

What we recommend doing now

• Inventory live journeys and triggered sends
• Identify subject lines driven by variables or lookups
• Check for nested AMPscript or stored personalization
• Add %%__SINGLE_RESOLVE%% where required
• Republish every affected send

Addressing this now keeps it controlled. Waiting turns it into cleanup under pressure.