Selective Exchange Online send delays in OWA — 5–7 days for specific messages, others sent seconds before/after deliver instantly. MS support insists “client-side.” Stuck.

Hoping someone has seen this pattern before.

Setup:

•   Tenant: Exchange Online (M365)

•   Affected user: uses Outlook on the web (OWA) exclusively — no desktop Outlook

•   Multiple devices (home PC, office PC), multiple networks

•   No on-prem Exchange, no hybrid

Symptom:

•   Specific messages composed and sent in OWA arrive at the recipient 5–7 days later

•   Messages sent moments before and after the delayed ones, from the same OWA session, same device, same network, deliver in seconds

•   No bounce, no NDR, no visible error to the sender — message just shows up days later

•   Pattern is selective and intermittent, not consistent

•   No obvious common factor yet across the delayed messages (still investigating recipient domain, attachment type, size, subject keywords)

What’s been ruled out:

•   Not desktop Outlook — user is on OWA, so no local Outbox, cached mode, OST, PST, or add-ins

•   Not the client device — happens across multiple PCs and networks

•   Not the network — OWA hands off to EXO over HTTPS; a network issue would fail visibly, not silently delay individual messages by days

•   Outbox sequential-release argument doesn’t apply to OWA

What Microsoft is saying:

Tier 1 support has concluded the delay is “client-side, between the Send click and the server hand-off.” That conclusion doesn’t fit:

1.  OWA doesn’t queue locally — the message is handed to EXO immediately on Send

2.  A client-side issue can’t selectively hold one message for days while releasing the next one sent 30 seconds later