So, I need a way to transfer one Mac mail signature design to another computer. I don't know why this is so obtuse and hidden from users, but here's my issue. I'm using a Signature generator to produce our company's email signatures but we have users across most of the major platforms. Other mail clients seem to behave well, but pasting the signature generator's output in Mac Mail on Mojave fails, and the signature design is broken. All the mac's being targeted are running Mojave. For my process here, I have 1) copied the generated signature, 2) opened preferences in Mail, 3) created a new signature, 4) unclicked the default font option, closed the preferences tab. When I add the signature to a message the design is changed to the point of unusability. Alternatively, I have also tried the creating a placeholder signature method (by creating an empty signature with no default font, identifying the signature file in the library and the replacing the body tags and locking the file) as well as just copying over the signature files. Each method either produces bad results and it's unclear why (even worse changes to the design) or no results at all. So...how do I easily copy and edit mail signatures for multiple users.
A theoretical solution...
I find my Mac Mail signatures are stored in the folder at
~/Library/Mail/V6/MailData/Signatures. You may be able to copy the contents of the folder at this location from one computer to another.
Hit ⇧⌘G (or Go > Go to Folder... in the Finder) and enter
~/Library/Mail/V6/MailDataon the computer that has the signatures you wish to copy. Locate the "Signatures" folder, and copy it over to your target computer.
Then (on the target computer), navigate to the same
~/Library/Mail/V6/MailDatalocation, replace the Signatures folder (if present) with the version you copied.
Open up Mail and check if the signatures are there.
I haven't tested this, so it might not work/it might mess things up. Make sure you have a backup copy of the signatures folder from your target computer before overwriting it so you can revert if anything goes wrong.
If there are already custom signatures on the target computer, they will be overwritten.
Building on Michael Noguera's answer, I was able to transfer just one signature. Some file copying and maybe editing are required.
On the Mac where the signature already exists, look in
~/Library/Mail/V5/MailData/Signatures (yours may not be V5 but pick the most recent).
If the destination Mac has no signatures yet, just copy the entire file
AllSignatures.plist. If the destination has some and you want to preserve them, you need to go into the file
AllSignatures.plist, find the signature you want by looking at the
SignatureName field. Copy that entire XML
<dict> ... </dict> block to the destination Mac's
AllSignatures.plist file (your destination Mac may have a different version path, i.e. V7, but that's ok).
Now you need to copy the signature itself. In that dict block, there is a field named
SignatureUniqueID and the field after it is a long hex string. Look for a file in that directory with the same name as the hex string. Copy that to your destination Mac's Signatures folder.
Quit Mail and restart it. But you may not be quite done.
In my version of Mail (version 13.0), when I opened Preferences --> Signatures, I could see my copied signature in the All Signatures list but it was not associated with one of my mail accounts. I dragged the name of the signature from the middle column to the account name in the left column. Then, when I try to send a new mail from that account, it finally showed up in the possible signatures list.