I say not flatteringly but with sincere conviction.

我说话不是奉承，而是心悦诚服。

《新英汉大辞典》

The letter closed with sincere greetings which I did not think referred to me, but the warden was sincere about the issue.

这封信结尾真诚的问候我认为不是跟我说的，但是那个监狱长对待此问题倒是很真诚的。

youdao

Chua's tone is arrogant but filled just the same with bullseye observations, and I spent a long time trying to untangle the sincere from the deadpan.

蔡妈妈的语气中充满了傲慢，而且我花了很长时间试图从那种不动声色中去寻找一丝真诚。

youdao

All participants were able to understand sincere comments, but patients with frontotemporal dementia had trouble understanding lies and sarcasm.

WSJ: When Social Skills Are a Warning

Now you may not agree with the steps the Department and the Administration are taking to encourage innovation, but from my conversations with them, they are sincere in their desire to see this transformation occur.

FORBES: The Department of Education: Serious About Transformation

It is heartfelt and sincere expression of disappointment which one should sympathise with, but it is not an accurate comment on a general trend.

CNN: STORY HIGHLIGHTS