Signatures are fairly powerful, and quite often a quick way to include a template in your email.
It does get a little confusing as soon as you have more than one account or more than one signature.
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In Outlook, in the Tools, Options, on the Mail Format tab, in the signatures area, it allows you to select which account you’re selecting or creating signatures for.
Naturally, you can insert a signature manually, by virtue of the Insert menu’s Signature item.
As for when they get inserted automatically, it depends on your editor.
If you use Outlook as you default editor, then when you create a new message, it will always start with the signature for the default account. Replies and Forwards will, however, automatically get the signature for the account the message was received on.
If you use Word as your mail editor (which I generally do not recommend, feeling it as being overkill), there is added functionality … if you change the account that a message is sent on, the signature automatically changes.
I found out this, and more in this article: Outlook Tips & Techniques: Signature Doesn’t Appear After Changing an Open Message’s Sending Account out at Windows IT Pro.
Personally, I typically don’t have a default signature inserted, but manually add one, depending on what I’m doing. Not unlike this one:
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