The crash I've been seeing even on the MailKit sample app: Process: Mail [2744] Path: /System/Applications/Mail.app/Contents/MacOS/Mail Identifier: com.apple.mail Version: 16.0 (3731.700.6) Build Info: Mail_App-3731700006000000~4 Code Type: X86-64 (Native) Parent Process: launchd [1] User ID: 501 Date/Time: 2023-07-27 12:14:13.0114 +0200 OS Version: macOS 13.5 (22G74) Report Version: 12 Bridge OS Version: 7.6 (20P6072) Anonymous UUID: B6039EA8-CA31-5C41-EE33-6E114E55702D Time Awake Since Boot: 620 seconds System Integrity Protection: enabled Crashed Thread: 4 Dispatch queue: IMAPNetworkTaskHandler queue (QOS: BACKGROUND) Exception Type: EXC_CRASH (SIGABRT) Exception Codes: 0x0000000000000000, 0x0000000000000000 Termination Reason: Namespace SIGNAL, Code 6 Abort trap: 6 Terminating Process: Mail [2744] Application Specific Information: abort() called Does yours have a similar crash on the IMAPNetworkTaskHandler queue?

Yep, you can parse out the attachments from the MIME data but for the decodedMessage(forMessageData:) function, Apple still leaves it to you to rebuild the whole RFC822 message. In my case where I need to edit the text/html MIME data, I need to pull out the text/html then rebuild the entire RFC 822 to return to decodedMessage(forMessageData:).

I was having crash-on-start issues too but it doesn't seem to be consistently reproducible [macOS Ventura 13.4.1 (c) (22F770820d), Mail Version 16.0 (3731.600.7)]

Michael Tsai from SpamSieve wrote some thoughts about MailKit: https://mjtsai.com/blog/2021/06/23/mail-app-extensions/ If you want to see what functionalities MailKit offers, best to checkout Apple's sample app with Mail Extensions: https://developer.apple.com/documentation/mailkit/build_mail_app_extensions

Correction to the above, context isn't nil. It's empty (context.isEmpty == true). Why?

Still seems to be an issue on Xcode and iOS beta 5