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?