The first line of the message (you have to scroll way to the right) indicates that it is being blocked by Spamhaus, which is one of the many spam-identification "services", and which apparently your mail/postfix installation is configured to use.
EMAILX communicates directly with the SMTP server that you specify in its configuration file, whereas the Linux mail command is set up to use postfix as its local SMTP server, and that's where the spam filtering is taking place. So EMAILX is bypassing it.
Unfortunately I don't have any simple tips on configuring postfix, but I suspect that an internet search will lead you to instructions on editing its configuration to remove the Spamhaus filtering. Either that, or use the link provided in the error message to request that Spamhous take your server off its black list.
Another possibility, I suppose, would be to create a bash script that could replace the mail command, which instead launched an instance of A-Shell to use EMAILX to send the message.
On a related aside, a year or two ago we found that microsabio.com was on a couple of spam "services" blacklists, and it was a veritable nightmare to try to get off them. There are several websites which will run your domain against many spam filter/services and let you know which if any have you tagged. But getting off them is a different matter. It's almost like a protection racket, where you have to become a priority subscriber/member to each of these services in order to request that you be removed from their lists. We eventually contracted with a guy whose business was to deal with this problem, and for a couple hundred dollars, he did manage to clear up the problem. (Although it seems to resurface periodically, which is why we sometimes send mail using multiple email domains.)
