You say ‘popmail’ and then mention outgoing mail. “POP” is a protocol for incoming mail, not outgoing.
If your ISP is now blocking port 25 access except to their own outgoing mail servers, that’s not unusual at all. Most of the big ISPs have been doing it for years. It’s actually a good thing, because it prevents bot-infected PCs on their network from sending spam.
However, it does mean that if you run your own server and are accustomed to sending mail through it, you’ll have to switch your outgoing server to the ISP’s server instead. This could be an issue for sending newsletters and the like, so you may want to ask.