Silly question: which name would you use?

I'd use Pam. My own email is inconsistent and it annoys me, never mind other people. People write to me by my short name but officially I use my long name. If I were Pamela LastName I'd be happy for you to write to me as Pam.

Pam.

Somewhere along the way she wanted to be called Pam. Her legal name is Pamela. But she likes Pam. That is what I think.

I would use the signature name unless the person invited me to call them another name. IMO it's the safest thing to do (I may be oversensitive on this issue because people constantly use a nickname for my name even though I have never once used that name, and those people include our IT staff, so I could totally see them setting up an email account with the nickname, ugh.

Pam
But I’d assume either one is ok and not over think it.

I personally would use the signature name the first time and kind of feel it out after that.  Typically, at least where I worked, e-mail addresses are system generated with your profile. If you want to change it you need to submit a request, most do not want to go through the process so just leave it as is.  For a first time e-mail I bet most people just use their signature at the close of  message. Usually after that you can determine what they go by or you can ask.

I’d say Pam since that’s what her email is directed to.

What drives me batty is when people use my first and middle names together when I literally NEVER use my middle name except when required to on legal documents. I’ve never loved my middle name but it’s one of those names that people like to roll in so they do (think Sue Ann). It’s annoying as hell. I have to correct people constantly that Firstname MiddleName is NOT how I want to be addressed. I wish I could use my middle initial on more things so people would Just.STOP.Already!

I'd use the signature name.

The email address is set by the company, so she may not have had any say in how it was set up. The signature is set by the person. Therefore I would use Pamela, as per her signature.

That was apparently a thing when DH’s mom and dad got married, but it wasn’t as in fashion anymore by the time we got married. Plus it would have really hurt the close relative I was named after. 🤷🏻‍♀️

I always use the signature line until I hear otherwise.

I would not go by the email address. As an example, I have an employee named Jane Doe. Our company has two Jane Does. My employee's email addy is JaneDoe2@email.
I wouldn't assume she goes by what is in her work email addy, but I would think she had a hand in her email signature.

DH only ever goes by his full first name, including for his byline, but his work email address was created by IT and for some inexplicable reason they've used a shortened version of his first name together with his surname.  He's asked for it to be corrected many times but IT keeps insisting it's either impractical or impossible to change it.

This semi-regularly results in people addressing him by this shortened version, not just in emails but also if they're then talking to him on the phone or in person.

Of course he's polite with them about it, even though it can be like fingernails on a blackboard sometimes, but the bottom line is they're mistaken in thinking it's what he prefers.

And I never go by the shortened version of my name either. 

Pamela since that’s her signature name.

Absolutely signature name. My work emsil is my initials. I wouldn’t want anyone referring to me that way. My signature is what I want people to call me, that is why I set it up that way.