using 'Hi' instead of 'Dear' in work emails...

[Crosbybeach]

...when you don't know the person you are emailing/responding to. Also if they have used 'Dear your name' to respond with Hi, when you don't know them just seems too casual.

I don't have a problem with people emailing me with 'Hi Crosby' when I don't know them. But if I don't know someone and it's a formal email, I'll always use 'dear'.

Am I hopelessly out of touch? (mid 40s...)

Memos were/are usually used for internal communication.

The standard format is very much like an email.