It's the same on the sender side. Most people of course just outsource it to some SaaS like Sendgrid, and of course have some fancy microservice event bus architecture to get it there. That 'your login email has been sent' actually means 'your email has entered the very first queue, and we're hoping it makes it through all the layers soon'.

There have been plenty of instances where I tried to log in somewhere, and the first attempt to contact my mail server was twenty minutes later. And of course they then deliver all five retries at once.