The addresses, in your contacts

Every reminder address MailWhen answers to, as a file your address book can import. Your mail app then completes them as you type, so 18mo@ is there without your having to remember where the range stops.

Three files, because every address book wants a different one. Pick the row that matches where you keep your contacts.

The files

If you use	Download	How
iPhone, iPad or Apple Contacts	vCard	Tap it, then Add All Contacts. Nothing else to read.
Gmail or Google Workspace	Google Contacts CSV	Four steps, on a computer
Microsoft Outlook	Microsoft Outlook CSV	Six steps, one of them easy to miss

The two CSVs are not interchangeable. They hold the same addresses, but Google and Outlook disagree about what the email column is called — and both importers ignore a column they do not recognise rather than complaining, so the wrong file imports cleanly and produces nothing at all.

The vCard is the only one that works by tapping. It is also the one Outlook cannot read: given a file of several hundred cards, Outlook takes the first and silently drops the rest. That is covered in the Outlook guide.

What gets added

Every reminder address the service answers to, which is more than three hundred. That sounds like a lot to put in an address book, and it would be if they had names.

The schedule addresses have no name at all. A nameless contact is only ever matched on its own address, so typing 3d finds 3d@ and none of them ever appears among the results for a person you were actually looking for.
Three carry names, all beginning MailWhen, so typing mailwhen finds those and nothing else: cancel@ as MailWhen Cancel, list@ as MailWhen List, and hello@ as MailWhen Help, which reaches a person.
The timezone addresses are not included. There are several hundred of them and they are made of place names, so they would turn up whenever you searched your contacts for a person in London or New York. They are also not addresses anybody types twice — one message sets your zone and that is the end of it.

You never need any of the files. The addresses are built from a pattern, so 9d@, 3w@ and 18mo@ all work whether or not anything is saved. They are a convenience, and they are reversible.

Removing them

Both CSVs file every contact under a single group named MailWhen — a label in Google Contacts, a category in Outlook — so they can be found together and deleted in one action.

Elsewhere, searching your address book for @mailwhen.com finds the set. Deleting them changes nothing about your reminders: anything already scheduled still arrives.