It is possible to set an outgoing email as high priority (header: importance = high, [x-]priority = 1) using the Important button in the message editing toolbar. But there seems to be no way to set it as *low* priority (importance = low, [x-]priority = 5) per answers to [1], [2]. Some sort of importance scale - including, notably, “low priority” - is available in many (most?) other email clients, and IMHO would be quite useful in eMclient, too.
If someone sends me a “low priority” message, eMclient silently drops that bit of information, and also prevents me from replying in kind (same goes when importing old messages).
Email priorities have been around for ages, and (on a quick search) RFC-2156 standardized the high/normal/low levels some 20 years ago. If nothing else, why crack the door open for someone to quip that “Outlook always supported it” 