Hello @v-mengmli-msft ,
sorry for the late reply.
1. How do you get emails and use email in send email action? I get them from a sharepoint list that takes it from a power apps using the 365 microsoft connector
2. Could you please show some screenshots about key steps of your flow? please find it below:

this is the formula I have in the send to box: first(outputs('Search_for_users_(V2)')?['body/value'])?['Mail']
I use this formula cause I have multiple emails listed and I need to send a single email (separately to each one of them).
"I suggest that you set conditions for outputs of email you get. For example, the first condition is used to determine whether the number of eligible emails is equal to 1, if so, then send the email directly, if not, then further filter the eligible emails, for example, the email needs to contain smith." = I cannot use this solutions, we are a big company and I would have to list hundreds of cases.
"Or you can also use Combo box in PowerApps along with the following formula to select the correct ID.
Items of Combo Box" = I don't believe it would solve my issue cause the emails are corrected from the beginning is simply that Outlook choose another emails cause its really similar.