Sometimes Yahoo mail causes this issue. As per their documentation, it is a temporary issue and mail should go through when sent again after four hours. To avoid this error, have the contact person add your email address in his/her white-list.
WebCube Internet Marketing #4, 10625-107 Avenue, NW Edmonton, AB, T5H-0W5P1: 780.655.2210P2: email@example.com
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WebCube Internet Marketing 1095 McKenzie Ave #300 Victoria, BC, V8P 2L5P: firstname.lastname@example.org
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