Sender address was invalid : IDN Domain issue?

A few months ago, I started having issues sending emails with Apple Mail via Zoho email accounts on my iPhone. This drove me nuts since it worked just fine on my MacBook and iMac. After constantly checking the IMAP and SMTP server settings in the Mail settings on my iPhone, which were set up correctly according to Zoho. The advice of a user in Reddit pointed me in the right direction.

I went to the iCloud Mail settings on my iPhone and scrolled to the bottom, where I found the "OUTGOING MAIL SERVER" SMTP iCloud SMTP Server. Not only was the iCloud SMTP Server in there, but to my surprise, so was the Zoho SMTP server for my custom email domain – and it was turned off! After turning it on, everything was back to normal and my custom domain emails started sending again.

DTChristie wrote:

Did anyone notice whether this bug has been fixed by Apple yet?

Anyone who actually read the thread would know that it is not a bug, and Apple can’t fix it. This is a problem with the user-provided settings in the mail account, and is probably because the mail account has 2 factor authentication, which means that an “application specific” password must be created to use with a mail client other than one provided by them email provider.

Hey IzaacJ,

We're assume you've double-checked your email settings for setting up your One.com email account, but we wanted to be sure: Setting up Mail on iPhone (iOS 15) – Support | one.com

If everything on your device is correct, please contact Apple Support to look into this further with you: Get Support

All the best.

The most common reason you can receive but not send is you didn’t enter a password for the outgoing mail server in your account settings when creating the account. It says “optional”, but it really isn’t. When setting up an account manually you must set up incoming and outgoing servers as separate steps.

Helpful or not, it is the correct answer.

Actually, it may not be the only possible answer. The other is broken VPN. If you use delete the VPN profile in Settings/General/VPN & Device Management/VPN. Don't just turn it off; delete the profile.

Looks like even the latest iOS 17.1.2 mail app cannot handle IDNs properly.

I added zídek.ch to iCloud mail, as a hosted email domain. To do that, I had to use the punycode domain name xn--zdek-vpa.ch before it would allow me to add it.

Then when I check the iCloud mail ‘from’ addresses, the IDN is displayed, but not the punycode one.

When I try sending from the IDN name, it fails every time.

Hope apple fix this soon. The world has woken up to IDN - especially important for all character sets for the far east, but it looks like apple is asleep here.

Having the same issue. Email works with windows Mozilla Thunderbird and Android Gmail app but iPad mail application just says the sender address is incorrect (approx. translated from Finnish). And I'm using the punycode version of my domain. I can receive mail on iPad mail app but cannot send.

Gmail app on iPad works just fine. So I guess I'll be using that one then.

Hki IzaacJ,

Is the email address you're using a custom domain?

If so, do you have an @iCloud.com email address also set up in the Apple Mail app? If so, are you able to successfully send emails from that email address?

If not, this page advises that you need to have that set up, in order to use a custom domain:

Set up a custom email domain with iCloud Mail on iPhone - Apple Support

When you subscribe to iCloud+, you can add up to five custom email domains and create email addresses for those domains on your iPhone (iOS 15.4 or later). Then, you can send and receive mail in the Mail app  on your iPhone, iPad, iPod touch, and Mac, in your Mail account on iCloud.com, and in the email app on a Windows computer.

You need to have a primary iCloud Mail address before you can add a custom email domain on iPhone. See Create a primary email address for iCloud Mail in the iCloud User Guide.

Take care!

Then some other SMTP setting is wrong. Port, SSL setting, server name (incoming and outgoing servers are not always the same), authentication type.

Sadly not. I think this is a case of waiting until Apple pulls its finger out and makes the fix. :-/

I also have the problem that sending mails having an IDN domain stopped working. All the settings are correct. Recreating the account has no effect: the process works fine, but mails cannot be sent.

This problem does not occur on macOS, just iOS.

@Lawrence: Your comment was not helpful. The email address with IDN works on macOS, but not iOS, with the same settings at the provider. Something has changed with a recent iOS update. Many people outside the US have addresses with special characters in them.