Why should I cause delays and give my users a slow website impression while 95% of internet sites deliver emails instantly? The internet has reached significant heights, and even 15 seconds for an email to arrive is considered too slow…
Not sure what Gmail SMTP is in this context. If a user sending mail has access to their diagnostics you might be able to determine if there is an issue by reviewing their logs if they choose to share them with you. Or if this is mail being sent by your website to your mailbox you can review your own MTA logs for errors or issues.