Update by Dimitar Nestorov May 2021
I just tried the sub+email trick and it works again.
(Not confirmed by OP)
Update June 2020
So Apple has decided in their infinite power trip to no longer allow the previous sub+email trick to work.
Big thank you to Josef Grunig in the comments for pointing out a new workaround, at least in Gmail.
You can still use the "." trick with gmail accounts: name.surname@gmail.com or namesurname@gmail.com without dot is the same account for Gmail while they are different for Apple Sandbox. Just tried and it worked.
This will allow you to have "multiple" email accounts all going to the same email, since you can put the "." anywhere in the email string before the @.
Example:
s.tandardappleid@apple.com
st.andardappleid@apple.com
sta.ndardappleid@apple.com
stan.dardappleid@apple.com
and so on, and Apple hopefully won't be able to kill this workaround since periods are allowed in email addresses normally.
If any other email providers allow this kind of functionality then feel free to include it in the comments.
The following has been left for historical record
Currently you cannot. The closest you can get is to use an email sub+address, if your email allows it. Gmail does for sure, others probably do too.
So if your normal itunes email is email@gmail.com, then you could set up a sandbox user like email+ios1@gmail.com and then do all the email confirmation and stuff in your normal inbox. This is useful because you don't have to create actual brand new address for the sandbox account.
This is still a huge pain though because itunes sees it as a seperate, unique account from your normal itunes account, and so if you want to try out the sandboxed app, you have to log out of itunes, log in with the new sub addressed account, and then when you are done testing you have to log out of the sub account and log back in with your normal account to have access to all your other apps.
Update April 2017
So I contacted Apple developer support, and after finally getting through their horrible Contact Us system, I got this reply back:
Thank you for reporting your sandbox issue.
Aliasing for sandbox testers is allowed if the main email is already registered in our system as being only a sandbox Apple ID.
The following is an example.
If standardappleid@apple.com is already registered as a regular account, you can not add standardappleid+alias@apple.com as a sandbox tester.
We will not be able to fix this for you as it is expected behavior.
I know it was working with normal apple id email addresses in Feb 2017. So now it looks like you'll have to have one address that is not associated with any Apple account in order to use sub addressing.
EDIT
Just to clarify a little more. If you use email@gmail.com as your main apple id, you can no longer use any variations of it with sandbox testing accounts.
Instead, Apple wants you to make a complete new and unnecessary email account such as email1@gmail.com that hasn't been used as an apple ID, and THEN use that one as your base email for the subaddressing. So once making email1@gmail.com, you could register email1+ios1@gmail.com, and email1+ios2@gmail.com, etc.
Best Answer
@sarnold Turning your comment into an answer because it worked for me.
Two Separate Websites
Apparently for historical and organizational reasons, Apple operates their developer and app store business in a bifurcated manner.
developer.apple.com
The web site for all the technical resources, including documentation, WWDC videos and such.
itunesconnect.apple.com
This completely separate web site handles the business end:
Problem: Different Handling Of Accounts
The developer.apple.com site is slick in that a programmer can have a single Apple ID used to join multiple developer accounts. She may work for three different companies, and be assigned a role on each of those companies’ Developer accounts all on the same Apple ID. When she logs in to the Developer site, she is presented with a popup menu asking which of the three companies’ Developer accounts she wants to access during this work session. Nice.
The problem: in iTunesConnect, not so nice. In iTunesConnect, the "admin" person cannot assign existing Apple IDs as members with a role. Very strange. The admin is forced to create a new ID for each person being adding to the team. That means the person joining must have multiple email address. If an admin tries to add you to their iTunesConnect with an email address already used on some other company's iTunesAccount, an error message reports that email address / Apple ID is already in use. Obviously the programming team running the iTunesConnect site could use some help from the Developer site's team. ☹
Workaround: Email Address Trick
The workaround cited by sarnold involves a feature of email addresses. The spec for email servers has a feature where you can extend your email address. You can add a suffix to the first part of your email name by appending a "+" PLUS SIGN. From what I could figure out, the email software first looks for the extended name. If no such name is found, it drops the extension and looks again. If found, the shorter version of the email address is actually used.
So if the programmer Susan wants to use her email address
susan@example.com
for a second or third iTunesAccount, she tells her client's admin person at Acme Corporation to use something like this as her email address:susan+acme.com@example.com
. Apple will still create a needless extra Apple ID for Susan, but at least Susan does not have to bother with creating and accessing extra email accounts. The emails sent by Apple will arrive at hersusan@example.com
address.Susan double-checked this would work before talking to the admin person. She sent herself an email to
susan+acme.com@example.com
to make sure it arrived at thesusan@example.com
account.Effectively, the email servers are not fooled by the extended email address, but iTunesConnect is fooled into creating an new Apple ID using an old email account.