IP Networking

We're in the process of testing a mail server that we hope will replace our current one that's being hosted by our ISP. We learned a few things along the way and would like to avoid them if possible. The biggest hurdle is getting around port 25 (SMTP). Our work force is approx 75% consultants who work from clients' sites. Many of these places block port 25, therefore our users can't connect to send emails.

I know it varies from site to site depending on what their IT dept puts in place, but is there a range of port that's safe to use? I know comcast blocks port 25 but I read there's a workaround with using port 587 to get around this. For those who have dealt with this, what solution did you come up with?

You could use SMTPS on port 465 instead, but be aware that if your server should work as an internet MX, port 25 has to be reachable, since there's no way to embed port information in the DNS record.