fatal: ?Bad msgno X in mail_elt, nmsgs = Y

[Brian Redman]

What's the recommended way to guard against trying to access a message 
via msgno that's greater than nmsgs.  I currently check myStream->nmsgs 
but occasionally when I mail_fetchenvelope(myStream, msgno) I get the 
fatal error anyway.

I know other mail clients may be modifying the mailbox while I'm 
monitoring it readonly and I'd be happy to get an error return but as 
it stands it's fatal.  So I assume there's a way to avoid the fatal 
error.  Is there a way to lock the stream or something so it doesn't 
get updated in the course of the mail_fetchenvelope()?

Thanks.
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