Besides, by the way: Mà … at the start

At the start of a sentence, Mà moves the talk on. It adds a new point, or turns to something you've just remembered, like "by the way" or "anyway": Mà mai em rảnh không? By the way, are you free tomorrow? Mà nè makes the change of subject clearer: Mà nè, anh Tuấn dạo này sao rồi? Oh, by the way, how's Tuấn doing? In Hà Nội that's Mà này.

It can also add one more reason, like "and besides", often with cũng before the verb: Thôi, đừng đi. Mà trời cũng sắp mưa rồi. Don't go. Besides, it's about to rain.

Mà thôi drops what you were saying, like "never mind": Mà thôi, kệ đi. Oh, never mind, forget it.

When the new sentence pushes against the last one, Mà simply means "but", like Nhưng mà. At the end of a sentence it insists instead: Em nói rồi mà! I told you! This Mà is casual, for talk and messages; in writing, use ngoài ra (besides) or nhân tiện (by the way).