Some things in macOS never change. For example, every so often a mail system which has been running happily suddenly decides that it’s not going to work any more. Although it might stop receiving mail, most commonly the problem is that it can’t send any. This article considers how you can find out what’s causing that, and how to fix it.

For something so everyday, mail is surprisingly complex, and for it to work properly there are many different systems which must be correctly set up. Before you go any further, satisfy yourself that this is only a problem with mail, and doesn’t affect any other Internet service. If it does, then you need to look at your networking more generally.

The next question is whether the remote server is running normally. Most ISPs have a status page, and Apple’s is here. If your service is claimed to be running well, that’s no guarantee, of course, but if it’s down then there’s little point in doing anything further until normal service has resumed.

You also need to be clear which app and service isn’t working. For most users with Apple’s Mail app connected through iCloud, your Mac doesn’t connect directly with Apple’s SMTP servers, but works through iCloud. If you’re unsure, open the Apple ID pane and check that Mail appears in the list of apps and is ticked.

If you’re using a third-party mail client, rather than Apple’s Mail app, then that’s different too. Apple provides full information about the required settings in this article, and you’ll need to refer to your app’s documentation to see how that should be done.

There are two special cases of failure which may not be obvious, but which can readily catch you out, even though you may have done nothing to change your mail client’s settings.

While Apple’s Mail app doesn’t do anything obtrusive to connect to its SMTP service or iCloud, third-party apps have to access the former using an app-specific password, generated through the iCloud website. If you use such a mail client, you probably set this up and forgot about it, and sometimes those apps seem to forget the app-specific password too, as I’ve explained here. Apple details app-specific passwords here, and you may need to remind your mail client of the password it should be using, or even generate a replacement.

Those using Apple’s Mail app to connect direct to an SMTP server (rather than with iCloud) may face a rather odd problem which rears its head every now and again: corruption in its SMTP server settings. Although I’ve never liked Mail, and seldom use it, it has one exceptionally helpful feature, its Connection Doctor, opened from the Window menu.

Whenever you encounter any problem with Mail connections for incoming or outgoing messages, open Connection Doctor, select Log Connection Activity, show the Detail dropdown view, and click on the Check Again button to see what’s going wrong. If you see that the SMTP connection is failing, suspect that the server settings have become corrupted.

Before trying to fix that, ensure that you have a detailed record of all the SMTP server settings your accounts require. Then open Mail’s Preferences, select Accounts, then in that select the affected account. Click on the Server Settings tab, and you should see a popup menu for the Outgoing Mail Account. Open that menu, and select the last item, Edit SMTP Server List.

For anyone who has migrated their current Mac from a succession of previous Macs, this may come as a revelation. One of these SMTP server settings goes right back to when I ran Mac OS X Server on a Mac Pro, which must be at least a decade ago. Mail just never forgets, unless you tell it to. Select old and unused settings and click on the – button to delete them, and ensure that you also delete the one that’s causing trouble now. Then click OK to ensure the list is saved, open it again, click the + button and add your SMTP server details afresh.

With a bit of luck, your outgoing mail service should now work fine again.