12. (10 points) Given the equation $a b c d=a a d \times e$, where different letters represent different digits, what is the maximum value of the four-digit number $a b c d$?

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【Analysis】In $aad \times e=a b c d$, the unit digit of $d \times e$ is still $d$
$$
\begin{array}{l}
(1 \sim 9) \times 1=(1 \sim 9) \\
(2,4,6,8) \times 6=(12,24,36,48) \\
5 \times(3,5,7,9)=(15,25,35,45)
\end{array}
$$

【Solution】From the above analysis, it can be seen that $e$ could be $1, 6, (3, 5, 7, 9)$. Suppose $e$ is 9, aiming for the maximum value, then $d$ is 5.

Checking from $a=(1 \sim 9)$, we get
$$
\begin{array}{l}
115 \times 9=1035 \\
225 \times 9=2025 \\
335 \times 9=3015 \\
\ldots
\end{array}
$$

Through checking,
$\therefore a b c d$'s maximum value is 3015.
Answer: The largest four-digit number is 3015.