Currently IIS is sending about 60k emails an hour.
Is this a good number?
How fast can IIS send?
Are there any good ways to increase speed?
Are there good alternatives for running on Windows?
What do you want to do...
Generally, any 3rd party software still has to put the emails over the network. The IIS SMTP server is good enough... if it's not, you need a specialist solution/server/OS...
Do you perceive this as a slowness? Do you have an actual need to send more e-mails in the same time? Then you should profile your system.
Have a look at the CPU & RAM usage on the server itself, and at your Internet connection's bandwidth utilization. If none of them is at 100%, then you don't have any bottleneck, and your "slowness" is due to how fast remote mail servers can accept your messages; you can't do anything about this.
If your servers has enough available resources even at peak utilization and you have free bandwidth, you can try increasing IIS's limit on outbound SMTP connections; this will allow it to talk to more remote servers at the same time, possibly increasing your throughput.
BTW, who is actually sending those e-mails? An application running on the server itself? An application running on another server? Your users? Are you sure there are more queued messages than IIS is able to send? Have a look at the mail queue, too; if it's empty or very low, then this is no IIS fault, it just doesn't have anything to send.