Using Mailgun, only 1 email being sent correctly, but with all other emails i'm getting an error.

This is the errror i am getting :

Client error response [url] https://api.mailgun.net/v3/sandboxxxxxxxxxxxxx.mailgun.org/messages.mime [status code] 400 [reason phrase] BAD REQUEST

I already verified account and email is being sent without error to one account but none to others. How can i fix this?and thanks.(all input is correct in configs and env files).

The working email(that gets no error) is the same email used to login to my account on Mailgun if that makes any difference.