Handle multiple recipients from headers

[elieux]

I'm trying out this sendmail for development and found a problem with using it through PHP's mail() function. The standard way to specify recipients seems to be to put them in the message headers and if there are more than one, separate them by commas. Unfortunate thing is that the format is not as simple as #21 proposes, specified in RFC 822 and RFC 1123 and witnessed for instance by this Go EML parser unit tests (but it doesn't cover many problematic cases either).

I don't mean to add complexity to mhsendmail, but without additional parsing net/smtp.SendMail() just fails with 501 Error: Bad recipient address syntax in this case.