3 of 7 links on my navigation menu are external links to others websites (same company but other services/not the same domain). Those websites are not the core of the business. Linking a navigation menu item to external URL is not good practice, is it? how can I solve this?
Navigation is always an interesting topic, and should be treated with care in each case. If you have a target group at hand, or a selection of your target audience – use them. You could use Card Sorting to find out what your users think of all your pages and links belonging. As a designer you are sometimes amazed how user group pages together into logical entities.
In your case, they might group all the external links into one logical entity and label it “External Resources”, then that would be your choice – adding an External Resources link to you navigation menu. The label could also be “Partners”, “Vendors”, “Manufacturers’” or the like.
But your users might also group in a different way, such as in sub groups. Let’s say you have Products as a top navigation item, and you have external links to the manufacturer of this particular product, then user might feel that this link should be on these specific product pages, way down in the navigation hierarchy.
It all boils down to how your target users group all your pages, and what your external links represent.
If the websites aren't the core of the business I would never place them in the main navigation.
I might support the idea of external links in the navigation if the other sites have the same design and you could easily navigate between all websites.
I think that most users don't pay attention to the address bar unless they want to switch site.
When we send people to another site of ours, or another environment that is sufficiently different to cause confusion, we hit them with an alert in a lightbox.
This technique might work best with the visual cue method suggested by @TJH .
By using the icon on the link, and then carrying that icon through to the alert box, you help users who don't want to leave the current site to identify future external links.
Since the external links are not the core of the business, I'd recommend not placing them in the menu next to the core business links, but instead under one logical entity, like the previous answer suggested.
One reason to place it in the same menu I can think of, is from a charity organization: most of the content was just data that the user wasn't supposed to interact with, like articles etc. Then later was added one simple page where users could log in and view and update their information. That was in different domain but the link was placed in the menu. So essentially it was their core business, and there was only one item so it didn't became too confusing.