Contain multiple external email addresses for Active Directory user?

Looking to use existing way or create a property on Person in Active Directory to contain multi-value strings of external email addresses, as to be indexed for quick searchability.
The idea behind it is we would have an outside system in place where a user would sign up at for something which would then in return create an AD account after it cross-checked some other unique information.
One thing it would check was whether their personal e-mail, like bobsmith121@yahoo.com or gmail or whatever, already existed for a user in the Active Directory database.
Is there a field that could be utilized for that?
If created what's the easiest way to do that?

Is there perhaps another system that integrates that could possibly even do away with that approach, and manage AD account creation/matching for us?
garryshapeAsked:
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Henrik JohanssonSystems engineerCommented:
If the mail shall be an incoming address for the user, just add it to ProxyAddresses property for the mailbox user in AD. ProxyAddresses attribute is multivalued. That will also cause that if some internal user try to mail to the address, it will go to the user's internal mailbox.

If you just wnat to have it as an additional string field, you can use CustomAttribute1..CustomAttribute15. We earlier used the method to have externalemail:xyx@domain in any of those fields to populate external address mapping in our external MX-servers (sendmail)
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garryshapeAuthor Commented:
Thought about proxy address but that could cause email issues sending to their addresses internally right?

Sending internally to their Gmail would show up in their Outlook because it goes to their Primary SMTP, which would be the on-premise Exchange email address they have.

Yeah this is more for just having a record of an external e-mail address they use to identify themselves for determining whether they need AD account or not, so I'm assuming AD user object is the best place to store that.
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garryshapeAuthor Commented:
I wonder if there's another area could be used rather than modifying the Schema
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garryshapeAuthor Commented:
I see some "extensionAttribute1" through "extenstionAttribute15", "Unicode String"
I think i can use those for this purpose
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Henrik JohanssonSystems engineerCommented:
I was doing too much PowerShell/Exchange when comment earlier.
The attributes are called CustomAttribute when using Exchange management. AD-attribute is ExtensionAttributeX
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garryshapeAuthor Commented:
Ok I think I understand now.
So CustomAttributeX in Exchange corresponds to ExtensionAttributeX in Active Directory?
Or are they independently unique
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garryshapeAuthor Commented:
Ok yeah that appears to be the case, looks great. thanks again for the tips
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Henrik JohanssonSystems engineerCommented:
Yes, it's the same attribute with different label. The attribute stored in AD is called ExtensionAttribute1, but Exchange labels it as CustomAttribute1
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