I've not had problems replying with Yahoo Mail but that doesn't mean your problem doesn't rest with Yahoo. It really sounds like a "user option" that's the culprit. After hitting "reply" notice what address is in the bar and make sure it's properly spelled and matches the return address of sender. Other than that... I dunno

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Nello Tare

1982 300D (266,001 mi.) looking and running great

Previously owned:
1976 450 SLC
1983 300D
1976 300D
also
1982 Jaguar XJ6 (loved, but gone)