I am not sure if this applies to Admin section or User section, but hope here are enough experinced users.

Today morning our colocation host had issues with one of BGP routers, rendering connection of 1st ISP with our colocation center barely usable. Packet loss was up to 45%, web pages loaded slowly or timed out, mail clients have had huge problems, only smaller mails passing thru, while mails with attachments of over 1MB did not pass at all.

ISP fixed the problem by changing failed router and problems seemd to be gone.

But since then most of users have problems with Outlook. Let's say, among 1000 users of out Zimbra server, there are approx:
- 10% using Zimbra Desktop
- 10% using Webmail
- 20% using Thunderbird via IMAP+SSL
- 60% using Outlook via IMAP+SSL

Those Outlook users now all have problems:
- Outlook can barely connect to Zimbra server, most of time synchronising and failing, going into Offline mode etc.
- IMAP messages are not synced with server, read mesages in Outlook are flaged as unread in Webmail
- MOST WEIRD are some mixed messages, where message has HEADERS of one message, and BODY from another. Never heard or seen something like this!

These symptoms are present within almost ALL Outlook clients since today morning connectivity problem!
And if during outage today problems were seen only within 2-3 companies within affected ISP, now the problem is spread among MOST of users, regardless of ISP.

All mail is perfectly OK when you connect to Webmail, but almost all Outlook clients are having problems.

Any idea where to start looking for problems?