Test that a welcome offer can't be claimed twice

Test that one person gets one welcome offer

Most email providers deliver name+anything@ and NAME@ to the same inbox, so to many apps one person looks like several new customers, each with a welcome discount. Sign up with each version and see whether your app noticed.

Updated 21 October 2026

The versions to try

Version	Example	Reaches
Plain	`k3f9x2@mailclub.dev`	The inbox
Plus-tag	`k3f9x2+promo1@mailclub.dev`	The same inbox
Capitals	`K3F9X2@mailclub.dev`	The same inbox

Any +tag works, and the inbox shows which version each email went to. Agents get the versions that deliver from get_address_variants.

Test it

Sign up for the offer with the plain address and note the coupon.
Sign up again with the plus-tag version, then with capitals, in a private window each time so no session carries over.
Look at the inbox. Each email shows which version it went to. If a second coupon came, its offer_reuse check fails and the Accounts page warns that the welcome offer went out more than once.

The check also says whether the coupons differ (the app issued a new one each time) or are the same shared code, where it’s up to checkout to refuse a second use. Record what checkout did with coupon results: a coupon accepted for a reused offer fails.

In a test

offer-reuse.spec.ts

const tagged = address.replace('@', '+promo1@');
let submitted = new Date();
await signUpForOffer(page, address);
await qa.waitForCode(address, { type: 'coupon', since: submitted, timeout: 600 });

submitted = new Date();
await signUpForOffer(page, tagged);
// The same person: no second coupon should come.
await qa.expectNoEmail(tagged, { since: submitted, type: 'code', subtype: 'coupon', timeout: 300 });
expect((await qa.offers(address)).passed).toBe(true);

Agents get the same with get_address_variants, wait_for_code (kind coupon) and check_offers, and the test_offer_reuse prompt walks through it.

Try it on your next sign-up test

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Questions

Do I need to switch anything on?

Yes, for now. Marketing and offer emails is in Labs: switch it on in the app under Account settings → Labs. It applies to your API keys and agents too; until then their marketing tools answer with an error that says so.

Do these versions really reach the same inbox?

Yes. Mail to name+promo1@ and NAME@ arrives at name@, as it does at Gmail, Outlook and most providers, so the test matches what real people can do.

What about Gmail's dots, like n.a.m.e@gmail.com?

Proxy Mail QA can't test those: on our domains, a dot makes a different address. If your app normalises Gmail addresses, test that rule with unit tests.

Should my app block +tags altogether?

Usually not: many people use them to sort mail. A common fix is to normalise the address when checking whether someone has had the offer, and keep the address they typed for sending.