Hi Im not sure this is the right forum. 

Im a slingshot customer and use their (what I would call) passthrough smtp relay.

Settings used with Thunderbird are smtp.slingshot.co.nz 587 STARTTLS no authentication. Which allows me to send from any email address normally.

This mostly works fine except occasionally and for some unknown reason I get bounced emails eg:

Diagnostic-Code: smtp; 550 permanent failure for one or more recipients (customer.care@pulseenergy.co.nz:blocked)

No matter what originator (I have several emails) the bounce would be consistent with the above until I changed SMTP settings/outgoing server eg to my gmail account.

So I did a search for free smtp provider and end up with SendinBlue and several others which are designed for mass emails.

I dont like to use gmail as relay as its slow and often modifies the originator email address to my gmail one (another wee problem).

I just want a reliable relay service without all the bells and whistles that lets me use any email originator address; any suggestions?

Thanks, Al.