Should an Under Secretary of Energy from the USA's Department of
Energy be addressed as "Honorable" (or Hon.) in the address of a
letter, e.g. Hon. John Jones?

Hi jimxogan,

In regard to an Under Secretary of any department in the US
government, the word "Honorable" should be used on the envelope and
inside address of a letter. However, to address this person, the word
"Dear" is used.

See here:

ADDRESSEE - Under Secretary of a Department

ENVELOPE AND INSIDE ADDRESS - The Honorable Jane S. Hart
Under Secretary of Labor Washington, D.C.

FORMAL SALUTATION - Dear Ms. Hart: 

INFORMAL SALUTATION - Dear Ms. Hart: 

FORMAL CLOSE - Very truly yours, 

INFORMAL CLOSE - Sincerely yours, 

Source: 
United States Government Officials
Secretaries USA: Forms of Address
http://secretariesusa.com/pages/formsofaddress/Formsaddress.html


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"Under Secretary" "form of address"
"Under Secretary" addressing honorable


I hope this is helpful. If you have any questions regarding my answer
please don't hesitate to ask before rating.

Best regards,
Rainbow

I refuse to use the honorific "dear' unless it's to someone form whom I care.
How silly to write Dear sir, unless you pay your account I'll (a) rip
your arms off (b) sue you (c) cry.

 I use "hullo...."  or "G'day..."  or "good morning..."