SMTP is transactional, so from the perspective of:
- your system: “the message is sent, the remote system queued it as ABC1234567” → “the message was handed off to the next responsible MTA”
- sendgrid: “the message was accepted and queued with queue ID ABC1234567”
What you don’t (can’t) see is the next step where another agent on sendgrid’s system goes through the queue, picks up your message, and hands it off to the next MTA.
The queue identifier helps with tracking - if it were to get lost you could go to sendgrid’s sysadmin and ask “what happened to queue ID ABC1234567 after your system accepted it?”