Using an ISP's SMTP is an incredibly obsolete and problematic concept. Poorly authenticated with even worse deliverability. It was a bad idea even 10 years ago

Using an ISP's SMTP is an incredibly obsolete and problematic concept. Poorly authenticated with even worse deliverability. It was a bad idea even 10 years ago and it's just horrid right now. Use your email provider's SMTP, even if it's you yourself.