I just started a new job and they forcefully apply an email signature to all outgoing Exchange email. The signature block is dynamic and picks up the senders name, phone number, email address, etc. from Active Directory and puts it in the signature. I sent myself a test email and saw that some of the information was wrong, so I contacted the email admin. He fixed it and asked me to test it again.

When I sent myself another test email it came through as plain text without any formatting. The company signature block is supposed to have formatting and a company logo. I was positive the email admin had messed things up. I was wrong.

Turns out the automated Exchange signature block is output in rich text (HTML) when the incoming message is rich text and is output in plain text when the incoming message is in plain text.

Mail.app has a setting that says whether messages will be composed as Rich Text or Plain Text. I had always assumed that applied to all new messages. Unfortunately it’s now apparent that the setting only means that a message can be sent as Rich Text. If you don’t include any formatting in your message then the message goes out as Plain Text, regardless of the composition setting.

The fix? Create a simple signature block with formatting. My first choice was a simple single-pixel line but Mail.app won’t accept a formatted line without any associated text. So now I just have the word ‘Thanks’ in bold.