If you are lucky (and if HTML formatting is very simple), you may be able to copy and paste a HTML signature directly into the Apple Mail Signatures box. Just remember to uncheck the "Always match my default message font" box.
Unfortunately, if HTML formatting is slightly more complicated, Apple Mail is likely to mess it up. In this case, follow the below steps to insert HTML code directly into the signature file.
Trying to install a fresh copy of macOS High Sierra on a MacBook Air using a bootable USB key. The installation fails to proceed with the following error:
This copy of the Install macOS High Sierra application is damaged, and can't be used to install macOS.
Read more: This copy of the Install macOS application is damaged
Microsoft Office 365 Exchange account stops working in Apple Mail on macOS. Apple Mail and Internet Accounts in System Preferences prompt to re-enter Microsoft Exchange account password, but after the correct password is entered, just shows spinning wheel icon indefinitely.
Read more: Microsoft Exchange account stops working in Apple Mail - Enter Password
Mac OS version of TeamViewer 15 displays the following message in Computers & Contacts tab:
You are currently offline
Please check your internet connection.
The application is trying to reconnect
The computer is actually online and TeamViewer can connect to clients by manually entering ID and Password. It's just Computers & Contacts section that doesn't work.
Read more: TeamViewer on Mac - You are currently offline. Please check your internet connection