To will default to SMTPAPI header now
To will default to SMTPAPI header now

sendgrid/sendgrid-go,sendgrid/sendgrid-go,karmakaze/sendgrid-go,VividCortex/sendgrid-go,karmakaze/sendgrid-go,ChannelMeter/sendgrid-go,VividCortex/sendgrid-go