Maybe you try to send from an eMail-adress that dosn't exist on your Server?

In most case you must send it from the domian where the site exist an the mailaccout must exist.

In the mail-configuration you can alternating select to send from individual server, account, password...

Email confirmation worked fine, both the buyer and me got emails about the order. Then one day, it stopped working, and since then no one gets the confirmation email. There was one time, when it worked for some unknown reason, but othern than that, it doesn't.

I didn't change anything, and yet, it stopped working. What's more, another website of mine, which I also made using Website X5, works fine, sends the emails all the time. The only difference between them is that they are hosted elsewhere.

So the question is, is it possible that the problem is somehow in Website X5, and I have to change some configuration to make it work, or is it the hosting server? (I'm using a gmail account on both sites)

If you don't made changes the page the only possibility in my opinion the hoster or gmail has something changed. That's logic, isn't it?