Multiple email accounts in mac mail signatures

Question

My boss uses Mac Mail with five different email accounts, each with an associated email signature.
When she composes an email, the "from" field defaults to whichever email account she used last, and its associated signature appears in the email. This is her issue since switching to Mavericks (not sure which version) : When she switches the "from" field from, say, her personal account to her legal practice account, both signatures appear in the email. In other words, the signatures don't switch when she switches accounts-- they just pile up at the end of the email.
Is there a setting that will allow her signatures to switch with the accounts, rather than her needing to go back and delete the additional signatures?

Answer 1

1. OK, so I see a menu of outgoing smtp servers. Does that replace the 'sent from' field or is it just using that server to send the message?
The SMTP server is unrelated to the 'from' address.
The pop-up menu referenced only exists if you have multiple accounts configured in Mail.app preferences - if you only have one account then there's no point in Mail.app offering the chance to choose which account to send from.
You should use Mail Preferences to create additional accounts, one for each email address you use. Each account can be linked to a specific SMTP and POP/IMAP server for sending and receiving mail for that address.
If you have multiple addresses that you use with a single SMTP/POP/IMAP account then you can use a single Account setup in preferences - just list each email address in a comma-delimited list in the 'email address' field. e.g. creating an account with 'email address' set to: '[email protected], [email protected]' will give you options for those two email addresses when you create a new message.