policy of your email provider, see also https://stackoverflow.com/a/58573839 (not a Perl problem)

perhaps they check only part of the username, and you use the wrong TLD (gmx.net/gmx.com/gmx.whatever), or maybe it's not exactly the same address, because of a typo or an invisible-and/or-unicode character

In reply to Re: problem sending mail by soonix
in thread problem sending mail by Anonymous Monk

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