On a Phone
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Most of the situations that call for a disposable address happen on a phone: airport wifi, a shop in a queue, an app that demands an account before showing anything. The mechanics are the same as on a computer, but two things make it easier to get wrong.
Never type the address by hand
The single biggest cause of a message that never arrives is a mistyped address on a small keyboard. One wrong character and the message goes somewhere nobody is watching.
Use the copy button. It puts the exact address on the clipboard, and pasting it into the other app takes one long press. This eliminates the entire category of problem and takes less time than typing.
If you find yourself needing to remember an address rather than paste it, type your own name into the field instead of accepting a random one. Something you will recognise later is much easier to retype correctly if you must.
Switching apps does not lose your mailbox
Leaving the browser to open another app is fine. The mailbox is tied to your browser, not to the tab staying in the foreground, and it is still there when you come back.
What does lose it:
Clearing browsing data. The identifier that links you to your mailbox lives in browser storage. Clearing it means the site no longer knows which mailbox is yours, and it will hand you a new one. The old mailbox and its contents expire on their own shortly afterwards.
Private browsing. A private tab discards its storage when you close it. Open the site in a normal tab if you need the address to survive.
A different browser. Your mailbox lives in the browser you created it in. Opening the site in another one gives you a different mailbox.
Waiting for a message
Return to the tab and leave it open. The inbox updates by itself as soon as anything arrives, so there is nothing to pull down or refresh. A verification code usually appears within seconds of being sent.
If the phone locks or the browser suspends the tab in the background, the connection may drop. Bringing the tab back to the foreground reconnects it and loads anything that arrived while it was asleep, so nothing is missed.
Reading a message
Tap it to open. The message opens inside the page in an isolated frame, which is why it can look slightly plainer than it would in a mail app. Scripts are disabled and the message cannot interact with the rest of the page, which is the point.
Wide marketing templates sometimes need horizontal scrolling on a narrow screen. That is the template's doing rather than ours: many are built for a desktop window and never tested anywhere smaller.
The mistake to avoid while waiting
Do not press for a new address because nothing has appeared yet. That deletes the mailbox the message is on its way to, and there is no way to get it back. Wait a minute or two, and if nothing arrives, read the page about codes that do not turn up rather than pressing the button.
Adding it to your home screen
If you use this often, most mobile browsers let you save a page to the home screen. It opens in the same browser context, so your mailbox is the one you already had, and it saves finding the site every time.