Envelope shows "Sent" but emails not delivered to any recipient — Developer Sandbox

Hi DocuSign team and community,

I'm running into a blocker on my Developer Sandbox account and would appreciate guidance, as this is preventing me from validating the multi-signer workflow before our production rollout.

The problem: I create an envelope from a template with 2 or 3 signers in sequential routing order. After sending, the envelope status shows as Sent in the envelope history, and the history confirms the email was dispatched to the first signer's correct address. However, the email never arrives. Not in Inbox, not in Spam, not in Promotions, not in any folder. Because routing is sequential, signers 2 and 3 are also never notified.

What I've already verified on my side: recipient email addresses are correct (double-checked character by character), tested with multiple real email providers (Gmail, Outlook, corporate domain) with the same result across all of them, checked Inbox, Spam, Promotions, All Mail and Trash, the envelope history confirms Sent status with the correct recipient email, I'm not sending clientUserId so recipients are not being treated as embedded/captive, and email notifications are enabled in account settings.

Before posting I also reviewed two existing threads that describe very similar behavior: 26865 (@Rob1_862 and nekoenko reporting demo envelopes marked as Sent but no email delivery in the past month) and 25664 (tanvirsaad and tml reporting that only the first signer in routing order receives mail, requiring manual Resend for the rest). In thread 25664, ​@Julie.Gordon  kindly confirmed there has been a recent update to the recipient email and pointed to the official article, and recommended opening a support case with a production API account ID to explore disabling it.

My situation seems related but slightly worse: in my case not even signer 1 receives the email, so the workflow never starts at all. That makes me wonder if I'm hitting the same recent update or a different issue entirely. Also, since I'm on a free Developer Sandbox account (no production account yet), the standard "open a support case with your production account ID" path doesn't apply to me.

My questions for the moderators and community:

1. Does the recent recipient-email update mentioned by Julie also affect the very first email to recipient 1 in a sequential routing order, or should that one always go out as before?

2. Is there any known sandbox-side throttling, deliverability change, or suppression that could explain zero delivery across multiple providers and fresh email addresses?

3. For developers on a free Developer Sandbox (no production account), what is the right channel to get email delivery logs checked on DocuSign's side? Is there a way to escalate this without a production API account ID?

4. Any settings I should double-check in the sandbox before this gets escalated?

For context, my environment is a Developer Sandbox (demo) account in the North America region (demo.docusign.net). I'm happy to share my Account ID, Base URI, failing envelope IDs and approximate send timestamps privately (via DM or whichever secure channel a moderator prefers) so the internal delivery logs can be checked.

This is currently the only blocker preventing me from finalizing the PoC and moving toward a production integration with DocuSign, so any official guidance would be highly appreciated.

Thanks in advance!