What you’re seeing
- A bounce containing 550 5.1.10 RESOLVER.ADR.RecipientNotFound, saying the recipient was not found by address lookup.
- The address definitely exists, and other people can email it without trouble.
- Mail sent by picking the name from Outlook's suggestions fails, while typing the full address works.
- It started after a migration, or after a mailbox was deleted and recreated.
Common causes
Every environment is different, but these are the causes our engineers find most often:
- The receiving Microsoft 365 tenant has no mailbox or contact holding that exact address, often because an alias was not carried over during migration.
- The mailbox was deleted and recreated, so cached entries in Outlook and the address book still point to the old object.
- The domain is set as fully authoritative in Microsoft 365 while some of its mailboxes still live on another system, so mail is never passed on.
- Directory sync has not created the cloud recipient yet, or an error is stopping it from syncing.
- Legacy address entries from the old system were not added to the new mailbox, so old internal references no longer resolve.
Why it matters
Bounces to addresses everyone knows are valid erode confidence in email quickly. Customer messages to sales or support aliases can disappear, internal approvals go missing, and staff start working around the problem with personal accounts. After a migration, these failures often affect shared or role-based addresses first, which are the ones customers use most. Because the bounce goes to the sender, you may not even hear about the lost message unless they take the trouble to tell you.
How AskTechGurus fixes it
Skip the trial and error. A certified engineer works on your actual system, finds the root cause, and fixes it properly.
Tell us what’s happening
Submit a ticket in about two minutes. Paste the error, describe what changed, and pick your tier. No charge to submit.
A certified engineer diagnoses it
A vetted, US-based systems engineer claims your ticket, usually within minutes during business hours, and investigates over a secure remote session you can watch and end at any time.
Fixed and verified with you
The engineer resolves the root cause and confirms with you that it’s working. You don’t just get a workaround, and you get a plain-English summary of what was wrong.
Pay only when it’s resolved
Fixed price from $99, invoiced only after you approve the fix. If it isn’t resolved, you don’t pay.
Frequently asked questions
Why does typing the address work but picking it from the list fails?
Outlook's suggestion list can remember an internal reference to the old mailbox rather than the plain email address. When that mailbox no longer exists, the cached entry fails while the typed address resolves normally.
Are we losing mail from customers too?
Possibly, if the missing address is one that outside senders use. We check which addresses are affected and whether external mail is bouncing as part of the session.