X500 cache bounced email Remote Server returned '550 5.1.11 problem

(below the error message)

One of our user (maybe more we don't know yet) can't send an email one a her contact (not one of our user but someone from another company)  It bounced back.  This external email recipient is not in her address book.  Besides it's not a contact in Exchange.  The autocomplete has been deleted, then we manually entered the email address of this recipient.  But it still boucing back.    The other users can send email to this recipient.  But for a reason or another it doesn't work for her.  

We also have this problem with external users from other company who a working in our office (those users have their own email address not related with us but we've created a external contact in Exchange otherwise it will bounce, but sometimes it doesn't work.

A X500 has been added for all users.  But it doesn't solve the problem.  

is it possible to just get rid of all X500 and the cache?  I can't believe we still have to manually add X500 address in Exchange 2016 or Exchange online.  X500 is used for emails between internal users I'm aware of it.  But it's very annoying.  

We already have this issue with some users (working for another office with their own emails for whom we've created contacts in Exchange 2016)

Remote Server returned '550 5.1.11 RESOLVER.ADR.ExRecipNotFound; Recipient not found by Exchange Legacy encapsulated email address lookup'

Thank you
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Jason CrawfordTransport NinjaCommented:
Add the following x500 value to the recipient in question as an alias to the mailbox:

x500:/o=exchangelabs/ou=exchange administrative group (fydibohf23spdlt)/cn=recipients/cn=userc4fba8ee

I'm guessing userC isn't the actual user so you will most likely have to convert the value yourself with the actual IMCEAEX address.  Below is a Technet article covering the conversion:


Optionally you can use this little script I wrote:

[string]$a = Read-Host "Enter the email address listed in the 5.1.1 NDR"
$b = $a -replace 'IMCEAEX-', 'x500:'
$c = $b.Split("@")[0]
$d = $c -replace "_", "/"
$e = $d -replace "\+20", " "
$f = $e -replace "\+28", "("
$g = $f -replace "\+29", ")"
$h = $g.ToLower()

Write-Host "The converted legacyExchangeDN is:`n`n$h" -ForegroundColor Green

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Zaheer IqbalTechnical Assurance & ImplementationCommented:

First question what has changed that may cause this issue ?
Has anything changed on your exchange ?
Anything changed on the recipients mail server ?
Can you send the the recipeient an email from gmail , hotmail etc..?
kt3zAuthor Commented:
Nothing has changed.  Of course over the years several migrations (from 2010 to 2013 to 2016)  Otherwise I don't see what could have changed.  

Nothing changed on the recipients server.  I can send an email to this user from my outlook (or OWA) but she can't.   We all have our mailbox in the same database on the same on premise Exchange 2016.  

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Zaheer IqbalTechnical Assurance & ImplementationCommented:
Ok does the user have cached exchange mode on ?
Can you send an email successfully using OWA ?
kt3zAuthor Commented:
Ok it's working from OWA.  But how can a get ride of the Exchange cache or switch it off  ?    thanks again
Zaheer IqbalTechnical Assurance & ImplementationCommented:
recreate the users mail profile for outlook.

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