Cannot Send Mail error message of not a valid email address

Cathy-0944 20 Reputation points
2026-08-19T20:16:41.1+00:00

I purchased my domain through Act Now Domains (who apparently has partnered with GoDaddy to support the accounts). A few years ago, they transferred the email functions to Microsoft. I am a sole proprietor business owner so I am both the Admin and the only individual on the account.

The mail account has been set up (as Microsoft Exchange) on my iPhone from the beginning and I had no problems until recently. Now when I try to email that account from another email account on my iPhone, it holds the message in the Outbox. When I attempt to resend it, I get the following message.

Cannot Send Mail

"my email address" is not a valid email address and has been rejected by the server. The message has been placed in your Outbox.

As I said, I have never had this problem before. I have tried multiple "troubleshooting" solutions, including deleting the mail account on my iPhone and adding it back in. Nothing works and finding help as the admin is next to impossible. I have been on a continuous loop with GoDaddy that is getting me nowhere. In the middle of a text conversation, they ask for me to validate the account, which I did more than five times. I have also sent them multiple test emails (per their request) but sending from that account is not the problem and after I do, they tell me they received it and that's where it stalls.

Has anyone encountered anything like this or have any ideas how to fix it? Thank you!

Microsoft 365 and Office | Subscription, account, billing | For business | Other
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Kai-L 19,060 Reputation points Microsoft External Staff Moderator
2026-08-24T22:51:08.2933333+00:00

Dear Cathy,

No need to apologise, the detail is genuinely helpful. I should be upfront that what you are describing does not look like a Microsoft 365 filtering problem. The "550 No Such User Here" rejection comes from the server that refuses the message, and that happens before the message ever reaches Microsoft. If Microsoft never received it, then blocked senders, quarantine, junk folders and mail flow rules cannot be the cause. That is also why nothing turned up in your spam folder. Your Exchange checks back that up. Your mailbox is listed, your name and business address are correct, and your domain shows as the only accepted domain. Everything on your side is configured as it should be.

The fact that every address you hold with that host produces the same 550 points to their platform still treating your domain as locally hosted, so it delivers internally instead of following your MX record to Microsoft 365. That would explain why their logs show the message as sent while nothing arrives.

Given how long this has gone on, it is worth opening a support case with Microsoft so an engineer can review your mail flow with you directly and confirm this in writing. Your message trace results will help them get there quickly. For guidance, see Get support - Microsoft 365 admin.

I hope this helps. Looking forward to your reply.

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  1. Kai-L 19,060 Reputation points Microsoft External Staff Moderator
    2026-08-19T23:08:57.31+00:00

    Dear Cathy,

    Thank you for the detailed update. The desktop test confirms this is not primarily an iPhone or Apple Mail issue. The 550 No Such User Here error means the sending mail server does not recognize your business email address as a valid recipient. Having the address saved in the sender’s contacts does not affect mail delivery.

    Since you receive other messages normally, the issue appears to be limited to this particular sending account or its email provider. Please test sending to your business address from an unrelated Gmail or Outlook.com account, and from the affected account using its webmail.

    As the Microsoft 365 administrator, please also check the following:

    1. Go to Microsoft 365 admin center > Users > Active users > Your account and confirm that the business address is listed as the primary address or an alias.
    2. Open the Exchange Admin Center and confirm the mailbox appears under Recipients > Mailboxes.
    3. Check Mail flow > Accepted domains and confirm the custom domain is present.
    4. Then run a message trace in the Exchange admin center for one of the failed messages. Message trace can show whether Microsoft 365 received, rejected, deferred, or delivered a message.

    Rather ask GoDaddy or Act Now Domains to send another test email, provide the full 550 error and ask:

    “My business email is hosted in Microsoft 365, but messages sent through your mail service receive ‘550 No Such User Here.’ Other external messages reach the mailbox successfully. Please check whether your server is treating my domain as locally hosted or retaining an obsolete mailbox or local delivery route instead of following the domain’s current Microsoft 365 MX record.”

    Also ask them to identify:

    • Which SMTP server generated the 550 No Such User Here response
    • Whether the domain is configured as local on their mail platform
    • Whether any old Workspace Email, cPanel mail, forwarding, or legacy Microsoft 365 configuration remains
    • Whether their server is following the domain’s current public MX record

    I hope this helps. Looking forward to your reply.

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  3. Cathy-0944 20 Reputation points
    2026-08-19T22:18:37.17+00:00

    Thank you for your kind understanding and your questions. You are right. It is beyond frustrating to be bounced all over the place.

    • The email address that iPhone is objecting to is my business email.
    • I just tried to send the email from the same sending email used on my iPhone but on my desktop instead. It did not send and had the error message SMTP Error (550): Failed to add recipient "business email address" (No Such User Here) -my note - that's also not true-my business email address is in the sender's account phone book
    • I am receiving all other emails on my iPhone that appear on my business account when accessing it on my desktop.
    • I only have three email accounts on my iPhone. And it appears to default to the sender address I used even when composing in the third account. For example Sender address used in original email to business email address is the default email in the Sent From in my iPhone. If I try to create a new email from third account to send to business email address, it shows Sender default address in the Sent From. I hope that makes sense.
    • Yes, I did try to manually enter the business email address and it still failed.
    • I did not get an error code on the one sent from my iPhone but as shown above in the 2nd bullet above, I received SMTP error 550 when attempted from my desktop.
    • These were steps I did from what I found from searching the problem:
    • Clear autofill: Delete the recipient's email address completely from the "To" field and type it out manually letter-by-letter instead of choosing it from the drop-down list.
    • Re-add account: Completely remove your email account from your iPhone and add it back fresh

    Again - thank you! I appreciate you. :-)

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  4. Kai-L 19,060 Reputation points Microsoft External Staff Moderator
    2026-08-19T21:05:40.9966667+00:00

    Dear Cathy,

    I am sorry for how drawn out this has become. Being bounced between providers with no resolution is exhausting, especially when you are the only person on the account. As a forum moderator, I am not able to inspect your mailbox, domain configuration, or message trace to diagnose and fix this directly. My role here is limited to providing general guidance and solutions for users to implement themselves.

    The first thing to establish is which address the iPhone is objecting to. The wording does not make clear whether your business address is being rejected as the recipient, or the other account as the sender, and those point to different causes. Could you confirm:

    • Is the address shown in the error your business recipient address, or the address of the sending account?
    • Does the same email reach the business address when sent from outlook or Gmail on a computer?
    • Can other people send mail to the business address?
    • Does the failure affect only one sending account on the iPhone?
    • If you manually type the full business address, rather than selecting the saved contact, does it still fail?
    • What is the complete error code, if one appears, such as 550 5.4.1 or 550 5.7.1?
    • Which troubleshooting steps have you already tried, beyond removing and re-adding the account?

    If it arrives from a computer, the problem is local to the phone, which would also explain why your provider keeps finding nothing wrong on their side. Once I have a clearer picture, I will be in a better position to point you in the right direction. Looking forward to your reply.

    Note: Please follow the steps in the forum documentation to enable e-mail notifications if you want to receive the related email notification for this thread.  

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