Complaining in person or by letter is generally more effective.

当面投诉或写信投诉通常更有效。

youdao

I'm writing this letter in purpose of complaining about your after-sale service.

我写这封信要投诉你们的售后服务。

youdao

Yet when you read this letter can you find Paul complaining?

然而，你读过这封书信之后，有没有发现保罗在抱怨呢？

youdao

Caldwell later fired off a complaining letter to the judge in the case, who admonished the parties to cut it out.

FORBES: Magazine Article

What a dreadful, complaining letter.

NPR: Part One

The next day he wrote a letter complaining about lack of cooperation.

WSJ: Cello's Golden Age Comes to a Close | J��nos Starker | By Stuart Isacoff