What's the problem Dale? You need to be more specific. I use Mac Mail all the time and it always functions for me.
There are dozens if not scores of such glitches. I could spend all morning describing them but one of the most frustrating is the access to a typical message from a listing such as "Sent" for instance. Instead of the original message what you get now is a display of all the original messages of which it's constituted, and what's even more annoying, all the cc:'s are missing
Of course you can get the original by accessing a Reply link but that risks the inadvertent transmission to all the recipients before you're even done composing the new massage
Incidentally another very annoying quirk of the new Mac Mail is the automatic "correction" of your spelling without your permission, without even leaving a flag. Thus "massage" in the previous parka which Mac substituted for my misspelling of "message" and by coincidence "parka" where "para" was intended. I've encountered this quirk so far hundreds of times, in many instances with the most absurd results
But to get back to message listings, the single most frustrating glitch is the disappearance of a message from a listing, e.g., "Sent". As soon as you incorporate one message in another (e.g. by simply replying to it), the original disappears from the listing and the only way you can recover it later is to laboriously scan each listing until you find the bone incorporating it
Incidentally "bone" was supposed to have been "one"