Hi,
It could well be. In fact, even no change wouldn't meant it doesn't come from it.
It's possible that the email server is configured with some rules to handle emails based on whitelists/blacklists/etc downloaded from 3rd party services. For example, this one:
mxtoolbox.com/blacklists.aspx
And thus, if something changed in that third party service, it could lead to your emails having problems without your server being changed in any way.
But you need to check the service your using to send the emails. You can see it in the settings of SparkMail and Outlook Express. It's not necessarily on your website hosting server. It could be an email server provided by your hosting, but which isn't managed by your own server (if you have your own server). Or it could be something totally different, like for example gmail if you're using a gmail address.