They may be using
spam.aserve.com.tw. as a spam filter. A proper email server will try
spam.aserve.com.tw. and if that fails then try
mail.aserve.com.tw.. Spambots are likely to try only one address. By using two addresses it is possible to use this connection sequence to filter some email. There may slightly delay email deliveries to them, but should not cause you email to be classified as spam.
SPF record indicates they may send email from either server. If you see email arriving from
spam.aserve.com.tw., this does not indicate that the message is spam.
If they use a database to track source addresses, it is possible to identify sending programs that do not follow the standard and connect to the mail servers in the correct order. How they handle senders that do not follow standards is up to them, but I would block all such senders that don't pass rDNS verification. Failed addresses can be agressively removed from the database. It would be appropriate to keep passing addresses in the database, flagged accordingly.
EDIT: Based on the behavior of their servers they may be failing mail delivered to the backup server
mail.aserve.com.tw. and accepting mail delivered to the preferred mail server
spam.aserve.com.tw.. This will cause spambots which send to backup mail servers to fail.