Find erroneous AD Connect sync object (cannot sync object)

Today I was faced with an error in ADConnect. It couldn't sync one of the contact objects from on premise AD to Azure AD (Office 365). The error was on the AADConnect log:

• Error in Connector operations
• Status:completed-export-error
• In error log (on the object)
• Error:
  Object TypeMismatch
• Connected data source error code:
  0x8023134a
• Detailed data source error:
  A object with same proxyaddress does already exist in Azure Active Directory, but have a objecttype that is not compatible (objectclasses: contact, group or user). Solve this issie in the local catalog services or in Azure Active Directory, and try again. 

After a lot of trouble shooting I found that it was a guest account in Azure AD that caused the error. A guest account is normally created when a user is inviting/sharing a Sharepoint site or document with a external user. These users show up as email_domain.com#EXT#yourdomain.com.

TO actually find objects with a specific email address in Azure AD and/or Exchange online you can do the following with PowerShell:

1. Start PowerShell
2. Connect to connect-msolservice
3. Connect to Exchange online
4. Run the script below (change the mail address)

Script

# Define email to search for
$mail = "rikard.strand@external.elkjop.no"

# Do the different searches (requires connect-msolservice)
Get-MsolGroup -All | where {$_.ProxyAddresses -match $mail } 
Get-Msoluser -All | where {$_.ProxyAddresses -match $mail } 
Get-Msoluser -ReturnDeletedUsers -All | where {$_.ProxyAddresses -match $mail } 
Get-MsolContact -All | where {$_.EmailAddress -match $mail } 

# Do the different searches (requires connection to Exchange online)
Get-Group -ResultSize Unlimited | where {$_.WindowsEmailAddress -match $mail } 
Get-DistributionGroup | where {$_.EmailAddresses -match $mail } 
Get-Mailbox -ResultSize unlimited | where {$_.EmailAddresses -match $mail } 
Get-Mailbox -SoftDeletedMailbox | where {$_.EmailAddresses -match $mail } 
Get-MailUser -ResultSize unlimited | where {$_.EmailAddresses -match $mail } 
Get-User -ResultSize unlimited | where {$_.UserPrincipalName -match $mail } 
Get-User -ResultSize unlimited | where {$_.WindowsEmailAddress -match $mail } 
Get-MailContact -ResultSize Unlimited | where {$_.EmailAddresses -match $mail } 
Get-Recipient -ResultSize Unlimited | where {$_.EmailAddresses -match $mail } 
Get-MailPublicFolder -ResultSize unlimited | where {$_.EmailAddresses -match $mail }