Learn Hardcore Croatian: Pošalji mi e-mail večeras, molim te.

"Pošalji" is the 2nd person singular imperative of the perfective verb poslati (to send as a single, completed action). It’s used when you want one specific sending.
"Šalji" is the imperative of the imperfective slati and implies repeated/ongoing action, e.g., "Šalji mi e-mailove svaki tjedan."

Why is it "mi" and not "meni"?

Mi is the unstressed dative clitic meaning "to me" and is the normal choice in neutral sentences: "Pošalji mi e-mail."
Meni is the stressed form used for emphasis or contrast: "Pošalji e-mail meni, ne njemu." You might also front it for emphasis: "Meni pošalji e-mail."

Why not "Pošalji e-mail mi"? Where does "mi" go?

Croatian clitics like mi sit in the "second position" of the clause. They come right after the first stressed word: "Pošalji mi e-mail..."
If something else comes first, the clitic is still second: "Večeras mi pošalji e-mail..." or "Molim te, pošalji mi e-mail..."

Where can "večeras" go? Does word order change the meaning?

"Večeras" (this evening) is flexible:

"Pošalji mi e-mail večeras, molim te." (neutral)
"Večeras mi pošalji e-mail, molim te." (emphasis on time)
"Pošalji mi večeras e-mail, molim te." (also fine)
Meaning stays the same; moving "večeras" just changes focus.