Why is the subject line important?

The Subject line is the first thing people see. Many studies show that about thirty five to forty five percent of recipients open an email based on the subject line alone. A strong subject earns the open. A weak one loses it.

Originally all headers were limited to plain ASCII. When we wanted to use other languages or even emojis we needed MIME encoding. That is why you sometimes see subjects that look like this \=?UTF-8?B?8J+MgA==?=. That is RFC 2047 in action. It wraps non ASCII text so mail systems can carry it.

Fun detail. Re does not truly mean reply. It comes from Latin res which means the matter. So Re Your invoice simply means about your invoice. That is why it showed up in old office memos and then email inherited it.