I am having major issues with Outlook and Office365. Mail is sitting in my outbox and not sending. This only started to happen after I had implemented a VLAN infrastructure at my office.

I have setup multiple VLAN's in my office. My servers and workstations are on one VLAN and other VLAN's for other server hosting purposes, such as backups and voice.

HP1820 - SW 1 is connected to SW 2 via VLAN trunk

HP1820 - SW 2 has a connection from SW 1 and has the extra VLAN trunk to the Draytek 2952

HP1820 PoE - SW3 directly connected via VLAN trunk to the Draytek 2952

I have 3 HP 1820 switches and 1 of them is a PoE (SW3). The PoE's sole purpose is for VoiP and is directly connected to my Draytek 2952 via tagged port on the switch. All is working fine.

The other 2 HP 1820's are for the main network and i have around 5 VLAN's setup. SW1 and SW2 are connected via Port #24 via a VLAN Trunk.

On SW2 port #23 is connected to the Draytek via VLAN trunk also.

All VLAN connections have internet and able to communicate internally as well out.

I have one major big issue with Outlook. My mail is stuck and i have captured some Wireshark TCP captures.

Can someone please help me as i am baffled and confused as to why this is happening only with the connection to Office365.


Wireshark Capture #1

Wireshark Capture #2

  • I have solved the puzzle. MTU is the factor. – O.B Aug 15 '16 at 2:51
  • After hours of troubleshooting and I decided to read the whole entire manual of the switch i have uncovered that VLAN tags add an extra 22 bytes on top of the MTU size and therefore packet loss was the issue. I reduced my MTU to 1450 on my NIC and voila Office sends email with no issues – O.B Aug 15 '16 at 2:53

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