I've been unable to find account setup details for that email provider. I've found this, which is at best incomplete info for what appears to be very old email accounts: https://whirlpool.net.au/wiki/exetel_faq_email The error message you're getting would seem to indicate you're being blocked by the incoming server of a recipient having a Gmail address. Does the same happen when you send to non-Gmail addresses? This looks like a problem being caused by you using an old SMTP server that Gmail is blocking for not using modern features meant to prevent abuse. You won't be able to solve that problem without changing your email provider if that's the case…