Assume the following configuration:
| foo.com | A | 192.168.254.100 |
| *.foo.com | A | 192.168.254.100 |
| bar.foo.com | MX | mail.example.net. |
Logically, you would be able to resolve bar.foo.com to 192.168.254.100. However the wildcard record will only function for subdomains that have NO associated records. To be able to resolve bar.foo.com and send mail to it, you will need to add an A record pointing to your IP in addition to the MX record. Your working configuration will look similar to this:
| foo.com | A | 192.168.254.100 |
| *.foo.com | A | 192.168.254.100 |
| bar.foo.com | MX | mail.example.net. |
| bar.foo.com | A | 192.168.254.100 |