A "congratulatory email" is a short and simple message sent via email to express happiness and admiration towards someone for their achievements or good news. Full definition
If you're struggling to keep your resume both truthful and attractive to employers, she recommends you emphasize measures of your success (preferably quantitatively, but anecdotal evidence is good too); keep a record of congratulatory emails and recommendations you've received from past jobs; and, if all else fails and you're truly stuck, hire a professional résumé writer. (businessinsider.com)
Some folks respond better to a private congratulatory email. (canadianbusiness.com)