Hi, We are using offline address book for Exchange 2010 users. Since we are not all users using Exchange 2010, some user send a email to internel users using offline address book. We know that outlook will store the Email Address cache as Address type as "EX" and specify a "legacyexchangedn" for the offline address. When we are enable mailbox from offline address book user, The user legacyexchangedn will change, then outlook will send out email address using cache with old "legacyexchangedn" and get the return error as below: IMCEAEX- _O=FIRST+20ORGANIZATION_OU=EXCHANGE+20ADMINISTRATIVE+20GROUP+20+28FYDIBOHF23SPDLT+29_CN=RECIPIENTS_CN=13 4883d548f98a@company.com #550 5.1.1 RESOLVER.ADR.ExRecipNotFound; not found ## For example: Exchange user : user1@company.com Offline address book: user2@company.com (not exchange user) user1@company already sent out email to user2@company.com used outlook. Action: 1.delete user2@company.com mailcontact in offline address book 2.enable user2@company.com exchange mailbox. problem: user1@company.com can't send out email to user2@company.com use outlook. because user1@company.com use outlook cache with old "legacyexchangedn". I know two method to fix the problem. first : add X.500 address to user2@company.com second : delete user1@company.com outlook cache. or *.nk2 file windows. without these two method , is possible to make exchange offline address book as a simple address type as "SMTP" address or any other best soultion to fix this problem. Thanks.

Hi, That's the required steps which have to performed. Additionally, please ensure the Offline Address Book is updated without issue. Thanks AllenAllen Song

Hi, I ensure th OAB is updated. We have scheldue to update at 6:30AM and 1:15PM everyday. The OWA suecess update in outlook cilent. The mail issue is outlook keep the old mailcontact cache. then send to "EX" with old legacyexchangedn in echange server. Thanks

Hi, At first , please make sure the "LegacyExchangeDN" attribute of the mailbox database is right. Use ADSI Edit tool , Expand to ConfigurationàCN=ServicesàCN=Microsoft ExchangeàCN=organization nameàCN=Administrative GroupsàCN=Exchange Administrative Groupà CN=Database Availability GroupsàCN=Databases Edit all the properties of LegacyExchangeDN. If the "LegacyExchangeDN" attribute is still not used when the message is sent to internal recipients . To resolve this issue, download and then install Exchange Server 2010 Service Pack 1 (SP1) from the following Microsoft website: http://www.microsoft.com/downloads/en/details.aspx?FamilyID=50b32685-4356-49cc-8b37-d9c9d4ea3f5b Please remember to click Mark as Answer on the post that helps you, and to click Unmark as Answer if a marked post does not actually answer your question. This can be beneficial to other community members reading the thread.

Hi, I can send to internal recipent after add X.500 old LegacyExchangeDN. Without this method, can I set offline address book as "SMTP" tpye ,not in "Ex" Type? Thanks.

Hi. Was there ever an aswer to the question? I am also getting the same error with sending to one of my users. #550 5.1.1 RESOLVER.ADR.ExRecipNotFound; not found ##

I have the same problem, Started today with whole domain of one of our customers. Internal and other mail recipients are ok. It started to behave like that in the middle of the day, out of nowhere. An hour ago, I have sent the mail to the customer without any problem.