Open Apple Mail
Apple Mail does not let you paste raw HTML directly. We have to trick it by creating a dummy signature file and swapping the insides.
Open Apple Mail.
Go to Mail (top menu) > Settings (or Preferences) > Signatures.
Select your email account in the left sidebar.
Click the + button to create a new signature. Name it “HTML Signature”.
In the content box, type a placeholder word, like “REPLACEME”.
Crucial Step: Drag the new signature from the middle column onto your email account in the left column so it is associated with it.
Quit Apple Mail. (The app must be completely closed).
Swapping the Code
Open Finder.
In the top menu, click Go > Go to Folder… (or press Cmd + Shift + G).
Paste this path and hit Enter: ~/Library/Mail/
You will see folders like V10, V9, etc. Open the folder with the highest number (e.g., V10 or V11 for newer macOS).
Go to MailData > Signatures.
Look for the file ending in .mailsignature that was Modified Today (it should be the newest one).
Right-click that file > Open With > TextEdit.
The Surgical Part: You will see a block of metadata at the top (lines starting with Mime-Version, Content-Transfer-Encoding, etc.).
Do NOT touch the top metadata lines.
Look for <body> (or where your placeholder text “REPLACEME” is).
Delete everything below the metadata header block.
Paste your custom HTML code (from Phase 2) into the file.
Save the file (Cmd + S) and close TextEdit.
Locking the File
If you open Mail now, it might overwrite your work. You must lock the file.
In Finder, click the .mailsignature file you just edited.
Press Cmd + I (Get Info).
Check the box labeled Locked.
Close the Info window.
Verify
Open Apple Mail.
Go to Compose a new email.
Select your new signature.
Note: You might not see the image immediately in the compose window (remote images are sometimes blocked in drafts), or the formatting might look slightly different than the final result.
Send a test email to yourself (ideally to a Gmail account or your phone) to verify the logo loads and is not an attachment.