> It may seem a bit unusual

met someone a few weeks ago. They said their email address is @<german ISP>.de but half the time they're sending from @icloud.com, I guess because they have an phone from Apple and don't understand the settings

I volunteer at a library where people can come with tech questions. Just last week, someone asked to help sort out their email addresses because they have various, from Microsoft, from an ISP, and because they have an Apple phone of course Apple also pushed an icloud address onto them that they didn't want

Seems quite common to have a second sending address with how Apple is pushing its lock-in (email addresses are some of the hardest things to migrate). Though, also without that, I tend to think people typically migrate their primary email address at least once or twice in a lifetime, not counting business emails that are sometimes also allowed for reasonable amount of private use (not saying that's a good idea but people do it). Not that unusual