SP List – Create URL

[$AutomationMailbox] is the name of the SP List column that contains the email address of the mailbox

Create a Single Line of Text column or Hyperlink Column and add below code to the formatting

{
  "$schema": "https://developer.microsoft.com/json-schemas/sp/v2/column-formatting.schema.json",
  "elmType": "a",
  "txtContent": "Open Mailbox",
  "attributes": {
    "target": "_blank",
    "href": "='https://outlook.office.com/mail' + @currentField + '/' + [$AutomationMailbox]"
  }
}