SMTP servers and MX records

[shee...@hotmail.co.uk]

We run Exchange 2007 SP3. In our network, we also have application
servers that need to send mail (alerts, notifications, etc). We don't
want these connecting to Exchange directly, so they all relay via some
IIS SMTP servers we use.

The SMTP servername they configure in their application to send mail
to is smtp1.domain.com.

If I NSLOOKUP this, I get five IP addresses back > 192.168.1.2,
192.168.1.3, .....192..168.1.6

If I NSLOOKUP, set type=mx, and lookup smtp1.domain.com then I am
returned ten hosts, all with cost 10 records, representing the ten
SMTP servers we use.

My question is, when an application wants to send email to
smtp1.domain.com, does it check the A record or the MX record? How
does this actually work? The person that set it up has long since
left :(

I assume it sending email to the A record, so does anyone know why an
MX record for smtp1.domain.com would have been set up?