send-email: change the default value of sendmail.validate

The purpose of this configuration option is to prevent your emails from blowing up on SMTP servers (rather than Extended SMTP servers). However, I find it often confuses people whose patches are otherwise correct, and they don't know how to solve the issue.

I haven't seen an SMTP server in a very long time which doesn't support extended SMTP. The default behavior should probably change. If not, the error message should be more clear about action items to address the issue.

I'll send a patch around to change this shortly.

-- Drew DeVault