Warning: whenever you want to modify your signature, you have to re-do this entire process. and now the signature should have the same size as the text. Now try to send an email with this signature, open the email in any mail client like Gmail, Yahoo, Outlook, etc. Make sure that 'Use the same message format as the original message.' is unchecked !! Paste the signature (the one you sent to yourself from Outlook) Once you've received this email in Mail, select and 'copy' the signature
Save it, and send an email from Outlook with this signature to yourself (the computer where you use Mail)
Make sure the font and font size is the same as the one you set in Mail as your default font.(example: Arial 12) Write your signature, edit the fonts, sizes, etc.
Create a signature in Microsoft Outlook. Go to any computer with Microsoft Outlook.