I’m sorry to hear that your Mail app keeps crashing. This could be due to various reasons, such as corrupted messages, damaged preference files, outdated software, or incompatible email accounts. Here are some possible solutions you can try:
Update your Mac to the latest version of macOS . This may fix some bugs or compatibility issues with Mail. You can do this by going to System Preferences > Software Update.
Rebuild your mailboxes to fix any errors or inconsistencies in your messages. You can do this by selecting a mailbox and clicking Mailbox > Rebuild.
Delete any problematic messages that may cause Mail to crash when you try to open them. You can do this by opening Mail while pressing the Shift key, then dragging the Message Viewer pane until you only see the list of messages, then selecting and deleting the corrupted message.
Disable all your email accounts and then enable them one by one to find out which one is causing the problem. You can do this by going to System Preferences > Internet Accounts and unchecking the Mail option for each account, then testing Mail after turning on each account.
Delete the preference files for Mail and restart your Mac. This may reset some settings and preferences that may be causing the problem. You can do this by going to Finder > Go > Go to Folder and typing ~/Library/Preferences, then finding and deleting any files that start with com.apple.mail, then restarting your Mac.
I hope this helps. If none of these solutions work, you may need to reinstall macOS Mojave or contact Apple Support for further assistance.