At the end of an email, it is generally more common to write "Thanks and Regards" with both words capitalized. However, it's essential to note that email etiquette is not strict, and variations exist. Both "Thanks and Regards" and "Thanks and regards" are used, and the difference lies in the capitalization.
In a formal or professional email setting, it is more customary to capitalize both words, making it "Thanks and Regards." Capitalizing the words adds a touch of formality and professionalism to the closing. However, some individuals may use "Thanks and regards" with both words in lowercase for a more casual tone.
Ultimately, the choice between capitalizing or not capitalizing is a matter of personal preference and the level of formality you wish to convey. As long as you maintain a polite and respectful tone, either form is generally acceptable in most email communications.