How to Check the From, Sender and Reply-To Headers of an Email

To check the From and Reply-To headers, send a test email to a mailbox you own, open its full headers and paste them into the From and Reply-To header checker. It lists the mailboxes in From, Sender, Reply-To and Return-Path and checks them against RFC 5322.

What RFC 5322 says

A message has exactly one From header, listing one or more mailboxes. When From lists more than one, a Sender header with a single mailbox must be present. Reply-To is optional and a message carries at most one. The tool checks those counts and that each address is well formed.

Step 1: get the full headers

Open the test message and choose Show original or View source, the label depends on the mail client, then copy the header block.

Step 2: paste them in

Folded lines are joined for you. The tool lists each mailbox it found and puts its findings underneath.

Step 3: read the findings

Finding	What it means	What to do
Display name holds another address	The name shown is an address that is not the sender	Change the From name in your sending tool
Several From mailboxes, no Sender	Breaks RFC 5322	Add a Sender, or send from one mailbox
More than one Reply-To	Only one is allowed	Remove the extra
Reply-To on another domain	Allowed; replies go elsewhere	Confirm that is intended
Return-Path on another domain	Allowed; SPF is checked on it	Check alignment if you rely on SPF for DMARC

Display names that mislead

Readers see the display name before the address. A name such as billing@example.com in front of a different real address can look like impersonation, so the tool flags it. It is a practical warning rather than a rule in the standard.

Reply-To and Return-Path on another domain

Both are allowed. Replies often go to another mailbox, and Return-Path is the bounce address SPF is checked against, so for SPF to count towards DMARC the Return-Path domain must align with the From domain. The tool shows a note when the domains differ. The DMARC alignment checker goes further, and what DMARC alignment means explains why.

Related checks

For the list headers in a newsletter, see checking List-Id. For authentication results in one message, use the email header analyzer.

What this check cannot do

• It checks header syntax and counts, not whether the mailbox exists or the domain authenticates.
• Group syntax and obsolete address forms are not fully read.
• Domain matching treats a domain and its subdomains as one family and does not use the public suffix list.
• It follows RFC 5322 as written, without later updates.

Clean the list you are about to send to

The right From address still needs a clean list. Upload your list on the verify page: the first 100 email checks are free with a Google sign-in, no card. Each address comes back valid, risky or invalid with a reason, and nothing is sent to anyone on the list. For larger or recurring lists, see bulk email verification, list cleaning and pricing.

Check your headers now

Paste them into the From and Reply-To header checker.