Fix "mail: no address" on send due to non-empty address slice.

[stephenhyde]

Since the bcc, cc, to are string maps, it will default the get to a string slice of 1 item of empty string. This will fail the parse address on the send route.

to := m.Header.To()
cc := m.Header.Cc()
bcc := m.Header.Bcc()
all := make([]string, 0, len(to)+len(cc)+len(bcc))

Regardless of empty cc, bcc, or to, this all will default to a minimum of 3 items.

this will cause "address, err := mail.ParseAddress(all[i])" to return an err as all[i] will be of empty string.

[stephenhyde]

@veqryn

[veqryn]

Thanks!