JanKusanagi at

Well, 1.4.1 is old, but that already supports address verification (since 1.3.3).

If you're not getting a warning when following those non-pump.io (therefore, not valid in this context) accounts, I guess the servers they're on support the Webfinger standard, even if they're not Pump.io servers.

But seriously, don't bother trying to follow non-pump.io accounts.