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> Hola

Hi / Hello / Dear [name],

Hi everyone, / Dear all, o simplemente All,

Dear [Name] et al. ,

(Et al. significa ‘y otros’)

Si llevas tiempo sin estar en contacto con el destinatario, puedes añadir:

– I hope …

… this email finds you well.
… you are well.
… all is well.

> gracias por

– Thank you for…

… your prompt response / speedy reply.
… sending [what] over.
… your enquiry.
… your email.

> perdona por el retraso

I  apologise for the delay in getting…
… a response to you.
… back to you.

Un poco más formal:
– Please accept my sincere apologies for the delay in my response.

> te escribo para

– I´m writing to you regarding

Bastante menos formal pero no por ello menos común:
– Just a note to say that…
– Just a quick note to tell you about
– Quick one regarding
– I just wanted to drop you an email to …
– I have got a few queries regarding …

> siguiendo con

Si tu email sigue una conversación ya iniciada puedes arrancar con una frase tipo como las siguientes. Son todas diferentes formas de decir “Siguiendo con …” 

Carrying on
Following on
To follow up
Further to

… the subject of…
… our last telephone conversation (this morning / earlier today) …
… from your email below …
… the email chain below …

I refer to …
As discussed, …
In addition to