If you don't want to enable the custom sender domain "feature", please use a sensible default, e.g. the actual sender address wiki@wikimedia.org.
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See Also:
https://bugzilla.wikimedia.org/show_bug.cgi?id=56414
https://bugzilla.wikimedia.org/show_bug.cgi?id=56413
https://bugzilla.wikimedia.org/show_bug.cgi?id=46670