Courier encountered an issue with email subjects being truncated or rendered incorrectly in Microsoft Outlook due to the default character limit of 127 characters, which restricts support for non-UTF-8 characters. To solve this problem, they discovered that encoding email subjects in base64 and using a specific format (RFC 1342) allows the server to decode and render them correctly. However, they also encountered limitations with some email providers and had to implement a recursive encoding process to ensure compatibility. The solution involved checking for Microsoft Outlook as the culprit behind the issue, rather than assuming it was a local configuration problem, and using online tools such as UTF-8 email subject encoders to test and refine their approach.