Are Email Addresses Case Sensitive? Capitals and Duplicates

Email addresses are not case sensitive in normal use. Capitals after the @ never change where a message goes, and Gmail and Outlook ignore capitals before it. Mail to [email protected] reaches the inbox of [email protected].

The email standard leaves one gap: a receiving server may treat the part before the @ as case sensitive, though the standard advises against it.

The capitals that cause trouble sit in your own contact list, where two spellings of one customer can become two records.

Are email addresses case sensitive before and after the @?

Only the part before the @ can be case sensitive, and only when the receiving server is set up that way. That part is called the local part. The domain after the @ never depends on capitals.

The rules come from Request for Comments (RFC) documents, the standards behind email and DNS (Domain Name System).

Part of the address	Example	Can capitals change where mail goes?	Where the rule comes from
Domain, after the @	Example.co.uk	No	DNS standard: RFC 1035 (1987), clarified in RFC 4343 (2006)
Local part, before the @	Smith	Only if the receiving server chooses to, which the standard discourages	SMTP standard: RFC 5321 (2008), sections 2.4 and 4.1.2
The reserved name postmaster	[email protected]	No	RFC 5321, section 4.5.1, makes this one name case-insensitive

RFC 5321, the mail transfer standard, tells servers that pass a message on to keep the capitals before the @. On some hosts, it says, the user "smith" is different from the user "Smith".

The same section discourages relying on that difference, because it hinders mail between systems. Section 4.1.2 asks any host that receives mail to avoid case-sensitive mailboxes (RFC 5321).

The example is as old as SMTP (Simple Mail Transfer Protocol): RFC 821 gave the smith and Smith pair in August 1982. The advice against relying on it arrived with RFC 2821 in April 2001. The approved draft that will replace RFC 5321 keeps both (Datatracker).

Why capitals after the @ never count

DNS ignores case, and the domain is the part DNS looks up. RFC 1035 says every comparison of domain names is case-insensitive, without exception.

Our own domain shows it. Its MX (mail exchange) record lists the servers that take its mail. Here it is with the name typed three ways, as looked up on 25 October 2026:

$ dig +short MX mailgraf.com
1 aspmx.l.google.com.
5 alt1.aspmx.l.google.com.
...
$ dig +short MX MAILGRAF.COM
1 aspmx.l.google.COM.
5 alt1.aspmx.l.google.COM.
...
$ dig +short MX MailGraf.Com
1 aspmx.l.google.Com.
5 alt1.aspmx.l.google.Com.
...

All three returned the same five Google servers. In both capitalised queries, the server names came back as google.COM and google.Com, copying our spelling, though a caching resolver may return an earlier spelling. RFC 4343 explains this: an answer may point back to the name in the question to save space, and the match still ignores case.

Are email addresses case sensitive in Gmail and Outlook?

No. Both deliver to one inbox whatever capitals the sender types before the @.

Google's rules for a Gmail username allow letters (a-z), numbers (0-9) and full stops (Google). Capital letters are not on that list, which suggests no two Gmail accounts can differ by capitals alone.

Microsoft has no product page on this. On its Q&A forum in 2024, a Microsoft Community Support Specialist said most large providers, Outlook among them, ignore capitals in the username (Microsoft Q&A).

If a capitalised address bounces as an unknown mailbox at one company while the lower-case form gets through, that server is a rare case-sensitive one.

Lower-casing the whole address is safe at Gmail, at Outlook and at any server that follows RFC 5321's advice.

Dots and plus signs in Gmail addresses

Gmail ignores dots as well as capitals in @gmail.com addresses: johnsmith and john.smith reach one inbox (Google). On Gmail for a work or school domain, dots do change the address.

Google also describes plus addresses such as name+news@. Mail to them still arrives in the same inbox (Google Workspace). To your contact list they are still different spellings, since only Google's servers know they share an inbox.

Capitals in your contact list: duplicates and missed unsubscribes

A contact list goes wrong when one address is stored twice in different capitals. A tool that compares the exact letters counts Smith@ and smith@ as two people.

That person then gets every newsletter twice. Worse, an unsubscribe on one copy leaves the other copy active.

Under the UK GDPR, once someone objects to direct marketing, their data must no longer be used for it (Article 21(3)).

The Information Commission's Office (ICO) advises keeping a suppression list of people who opted out and checking new marketing lists against it (ICO). Each compliant unsubscribe feeds that list, and the check only works when it ignores capitals.

How to remove capital-letter duplicates in a spreadsheet

Lower-case every address in a helper column, then remove duplicates on it. Two functions do the work: LOWER, which turns capitals into lower case (Microsoft), and TRIM, which strips stray spaces (Microsoft). In Excel:

1. Copy the sheet first. Microsoft notes that Remove Duplicates deletes duplicate data permanently (Microsoft Support).
2. Add a column with =LOWER(TRIM(A2)), where column A holds the addresses, and fill it down.
3. Paste the new column over the old one as values.
4. Highlight the pairs with Home > Conditional Formatting > Highlight Cells Rules > Duplicate Values. If either copy of a pair unsubscribed, mark both rows as unsubscribed.
5. Choose Data > Remove Duplicates and tick only the email column.

In Google Sheets, check unsubscribes first. Data > Data clean-up > Remove duplicates then treats cells that differ only in letter case as duplicates (Google). An email verification pass afterwards finds addresses that no longer exist.

How MailGraf handles capital letters in addresses

MailGraf saves every address in lower case, from file imports, signup forms, the API (application programming interface) and manual entry. Both Smith spellings become one contact, and a capital letter alone is not counted as a repaired address.

In a file import, a second spelling in the same file is counted under Repeated in file. An address already in your account counts under Already in contacts.

Tick Update contacts that already exist and that contact is updated instead. A contact who unsubscribed stays unsubscribed, whatever capitals the file uses, and a block-listed address is counted under Block listed.

Gmail dot and plus variants are not merged: [email protected] and [email protected] stay two contacts.

Run your own spreadsheet through a first import after creating a MailGraf account, and its Duplicates tile shows how many copies it held, capital-letter ones included.