I do not understand what you are trying to say:
why can't i paste an email address... that I've copied using my mouse
I have no idea how it is possible to Copy an email address using only the mouse. As far as I know, you need to use the Copy command from the Edit menu or a shortcut for it. If you are not using the Copy command, you are not Copying.
Are you perhaps DRAGGING an email address from one field to another using the mouse? That only works for certain fields in certain situations.
Readers cannot see what you are doing. You will need to describe the simple steps you are taking in great detail, as if you were explaining to a child.
You don't know how to copy and paste using only the mouse?
On a Windows PC:
You highlight the address, right click and select Copy
You then put your cursor in the To, CC or BCC field, right click and select Paste
On a Mac everything is the same until you put your cursor in the To, CC or BCC and right click...nothing happens.
You have to use the keyboard to paste the address in the field.
Is there no way to cut the keyboard out of the equation?
Apple has whole departments that enforce the uniformity of User Experience across the way Applications work. So it was puzzling to me at first why these should behave differently from, say Cut and Paste in a Word Processor.
On closer examination, you generally are not selecting and cutting the middle six letters, or pasting with an insertion point into the middle of a field of letters. You are instead manipulating these email Addresses as "objects", and the drag-and-drop metaphor can be used to duplicate and re-use them.
If you were to instead take only the firstname out of an Address (which is difficult, as mail wants to select the email Address as an "object" in its entirety), and paste it into the middle of a different field that is actually text, I think the right-click options should work as you expected.
No offense taken. This forum format does not lend itself to subtlety in expression.