Trust that you are well.

[Gray]

"Trust that you are well."

How often this is used as an opening in the mail?

Is it common in UK?
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First lesson - English, not english. I, not i. ~A student of English

[Kitosdad]

Yes Gray, it is a normal opening in a letter.

Trust that you are well.
Trust that all goes well for/with you.
Trust that all is well within your family.

All are frequently used.
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Keep it simple ... Keep it interesting.

[Lackswords]

Trust that you are well.

I am new here. Would like to ask what are the more appropriate replies in response to the opening in a letter/email. Kindly advise please.

[Beeesneees]

Hello Lackswords,

Welcome to the forums.

With regard to the openers mentioned above, I would not use them without adding a subject pronoun so that the sentence is completed and it seems less like note format:
"I trust that you are well."

The appropriate replies to letters/e-mails vary according to the individual circumstances: what sort of letter it is, what the opening statements of the original letter are, how well you know the person writing, etc. Responses can range from the very general:
'Thank you for your letter'
'I was so pleased to hear from you.'
To responses to very specific comments within the letter.

Did you have a particular letter or letter opening in mind?
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Cheers m' dears!

[Lackswords]

Hello Beeesneees,

Thank you. In fact I received an business email from a business contact which opened with "I trust that you are well", followd by specific comments.

My question was triggered by the recent knowledge that the more appropriate response to "How do you do?" is "How do you do?". Just wondering whether there is something similar in written English.

[Beeesneees]

Hello Lackswords,

As you have discovered, general conversation openers such as 'How do you do?' are really polite formalities where the questioner does not really require an honest response - the last thing you should do is delve into how unwell you may be feeling, unless the questioner is genuinely concerned about some ailment. Incidentally, if you wish to answer the question rather than just ignore/echo it by saying 'How do you do?' in return, then is it sufficient to say something like:
'Very well, thanks, and how are you?' or 'I'm not doing badly, and how are things with you?'

The situation is slightly different with a letter/e-mail because it is not a true dialogue in the same way as a conversation. Rather it is a series of inter-linked monologues. My personal feeling is that in responding to the letter you should aim to ask something similar to show the same level of concern for the other person's well-being.

'I trust that you are well,' as an opener in a letter is, as you suspect, just like the case with 'How do you do?' A breakdown of your health is not required in response.. These could be considered appropriate responses (Some could also be used as sentences at the end of a letter):
Thank you for your best wishes. I trust that everything is also well with you (and your family).
I hope that everything is going well for you (and your family).
Thank you for your letter.
I was pleased to receive your letter and hope that all is well.
I hope that this letter finds you well. (This is formal and very little used nowadays - I know that the sentence structure -- the idea of a letter 'finding' someone --probably seems very strange to you, but an older native speaker would understand,)
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Cheers m' dears!

[Lackswords]

Hello Beeesneees,

Thank you. I hope this post finds you content (I have seen similar sentences in emails of my colleagues - including some younger native speakers - here in Hong Kong! :P)

Cheers,

[Beeesneees]

That's another option, Lackswords! ;)

By the way, I don't feel you 'lack words' at all. I think your English is excellent.
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Cheers m' dears!

[Lackswords]

Thank you for your kind words, teacher Beeesneees. Will keep working on it, especially spoken English as we are preparing for kindergarten interviews (the majority to be conducted in English).

By the way, seems

'I have seen similar sentences in emails of my colleagues - SOME OF WHOM ARE younger native speakers - here in Hong Kong!'

is better than

'I have seen similar sentences in emails of my colleagues - including some younger native speakers - here in Hong Kong!'

Cheers,