SendGrid "To Email" Question

Hi All,

Using the SendGrid plugin to trigger an email when a private message is received. I have this working in other areas and I tested this one with a specific email and it works. However, I’m trying to retrieve the email of a specific user (the user who is receiving the private message). I have it setup like this currently:

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So it takes this current “message thread users” emails and removes the creator of this new message, so in theory it should return with the other user’s email. I get this error when triggering the workflow:

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I’ve also tried this route:

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“who to notify” is a datatype of user which is used for trigger notifications but it should be the person that also gets the email, however I get the same error.

Is there a way I could get the right email and avoid the error? I was able to do this successfully on another part of my app with this condition:

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Does it HAVE to be a backend workflow or something? Thanks for any help.