Every time I try to launch mail it starts looks as if it it is loaded then switches off

The cause is usually a malformed email message, and complaints are growing increasingly common. It seems to affect older macOS versions, exactly how old is uncertain though. Presumably newer versions of Mail can cope with them while older ones can't.

Press and hold a Shift key while launching the Mail app again. That will launch the Mail app with no message selected. In that manner you may be able to identify the problematic message, and delete it.

If you are unable to find such a problematic message because Mail keeps quitting, the only recourse you may have is to use your email service provider's webmail interface (assuming they offer one) to delete any likely problematic messages.

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Don't be surprised if it happens again. It may even become more frequent, but now you know how to fix it.