The phrase "a supportive letter" is correct and usable in written English.
You can use it when referring to a letter that offers encouragement or assistance to someone.
Example: "I received a supportive letter from my friend during a difficult time, and it really lifted my spirits."
Alternatives: "an encouraging letter" or "a comforting letter."
So she wrote directly to the poet, who sent a supportive letter.
From then on he followed my career in an encouraging way, sending a supportive letter at a particular low point.
Ms. Montalta said she met Mrs. Baldwin last year after she mailed the fund a check and a supportive letter.
After he read aloud a supportive letter from Smith, the supervisors voted unanimously to approve the landfill location.
"A supportive letter from Mr Gove encouraging us to raise awareness of this issue in schools would help schools address this serious problem in a sensitive way.
The headteacher of her school had sent out a supportive letter informing parents that Meadows was transitioning to live as a woman.
November was actually Mental Health Awareness Month, hosted by us, and so we did a whole range of events, from a tabling event called Donuts and Notes, which is personally my favorite event because we just bring free Monuts to the BC plaza, and people can get a free donut for writing a nice, supportive letter to a friend.
In response to the bill, AHIP sent a broadly supportive letter last week but expressed concern about the changes to Medicaid.
The 63 character references stood in stark contrast to his brother's sentencing, when the judge in that case noted that he had not received a single supportive letter.
CISPA has the support of organizations that include AT&T, Facebook, IBM, Microsoft, Oracle, Symantec, the U.S. Chamber of Commerce, Verizon, with Facebook sending over a particularly supportive letter of endorsement.
When you conduct research with a professor or other senior scientist, she or he will be able to assess your skills (e.g., communication, problem solving, and interpersonal), thus giving them the chance to write a strong and supportive letter about your abilities as a potential scientist.
