Cross posting is almost always a bad idea, and something that you'll find many don't react very positively to seeing. There are cases where you can do it, but:
The question must be on-topic for all sites considered, according to the help center on each site. Don't judge what is or is not on-topic by the presence of questions about it - they might be old, or just not noticed enough to close when they were asked.
Tailor your question on each site to best fit that site's topic. If my question is about PHP and Wordpress, my Stack Overflow version of it is going to be more focused on the PHP aspects, and the Wordpress SE version of it would be more focused on the Wordpress specific aspects.
Essentially, if done right - you have not cross posted, but asked two similar but distinctly different questions on separate sites.
In your case, no - questions about how to configure a mail server are not on-topic for Stack Overflow. Some were asked and answered, but this was before we launched Server Fault.
If you don't get a good answer on a site devoted to the topic of the question, revise the question if possible (and/or) try another bounty - you can also increase your chances by sharing your question on Twitter, Facebook, etc.