The seed migration read config('mail.mailers.smtp.host') and its three
siblings directly. An install configuring SMTP through MAIL_URL (the
other form config/mail.php supports) never sets those individual keys —
MailManager only substitutes the URL's components in at transport-build
time, inside its own protected getConfig(). Read raw, that left the
migration seeding config/mail.php's own placeholder defaults, which the
UNCONFIGURED_HOST/UNCONFIGURED_PORT guards then read as genuinely unset:
blank host, port 0, an unconfigured mailbox, for an install that was
sending mail successfully.
resolveSmtpConfig() calls the same Illuminate\Support\ConfigurationUrlParser
MailManager::getConfig() itself delegates to, so this is the same
resolution rather than a second copy of its merge order. Verified against
the real (protected) getConfig() via reflection: the URL wins for
host/port/username/password wherever it actually supplies one, and an
unset MAIL_URL leaves this a no-op.
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