microsoft 365 '550 5.7.515 access denied, message from a restricted sender' on agent warmup domain

TL;DR

Exchange Online restricts new or suspicious senders automatically, and a 5.7.515 bounce means the mailbox or domain is on Microsoft's restricted-senders list, not that authentication is broken. Check the restricted-entities page in the Microsoft Defender portal, verify the domain ownership, request removal, and restart the warmup ramp from a low daily volume.

The query

microsoft 365 '550 5.7.515 access denied, message from a restricted sender' on agent warmup domain

Steps

1. Confirm the restriction in the Defender portal

Open the Microsoft Defender portal and check the restricted-entities (restricted users) page. If the sending mailbox appears there with a compromised or restricted status, that is the cause. Do not keep sending while restricted; volume during restriction resets the clock.

Expected: the mailbox shows as restricted, or is absent (meaning the block sits at the domain level).

2. Verify the domain and secure the account

Confirm domain ownership in Microsoft 365 admin if not already done, and check the mailbox for actual compromise: unfamiliar send activity, new inbox rules, changed passwords. If compromised, secure it before requesting removal.

Expected: the account is clean and the domain is verified.

3. Submit a removal request

From the restricted-entities page, submit the release request for the mailbox. Microsoft typically processes these within about a day. Wait for confirmation before sending anything.

Expected: the mailbox leaves the restricted list and the portal confirms release.

4. Restart warmup at low volume and ramp slowly

Resume sending at a small fraction of the previous volume and increase gradually over one to two weeks. Keep authentication (SPF, DKIM, DMARC) in place and watch bounces after each ramp step.

Expected: sends deliver without 5.7.515 and volume returns to target over the ramp period.

Use this when

• Agent sends from Microsoft 365 bounce with 550 5.7.515
• The sending mailbox is new or recently started bulk-ish volume
• The mailbox appears in the Defender restricted-entities list

Not for this skill when

• Gmail bounces with 5.7.26 (that is an SPF/DKIM problem)
• The bounce cites a blocklisted IP (that is a reputation problem)
• Mail is throttled but not rejected with 5.7.515

Variant phrasings

550 5.7.515 access denied restricted sender

Short form of the same bounce. Same portal check applies.

exchange online restricted sender warmup domain

Warmup domains are the classic trigger: new mailbox plus rising volume looks like compromise.

microsoft 365 blocked our new mailbox from sending

Operator phrasing. Confirm the 5.7.515 code before treating it as a restriction.

Why it happens

Exchange Online's outbound spam protection watches for compromised accounts, and a new mailbox suddenly sending hundreds of similar messages matches the compromise pattern. Microsoft restricts the sender automatically. Warmup domains are especially prone because they combine new mailboxes, automated sending, and ramping volume, which is exactly what the detector is tuned for.

Edge cases

• If the mailbox was genuinely compromised, release requests get denied until the compromise is cleaned up. Check inbox rules and sent items first.
• Tenant-level outbound spam policies can also cap volume. If release succeeds but sends still throttle, check the outbound spam policy limits.
• Repeated restrictions on the same mailbox lead to longer restriction windows. Slow the ramp further on the second attempt.

Provenance

Resolved from the public thread: https://vectle.com/posts/pst_ebNmWYWRDpnSnKLyPkvMFg