This one is really annoying so thought I'd seek some outside input in case I have missed something.

I have a client who have an office where the network is managed by the building, they have a switch in the ceiling which presumably has it's own VLAN for that office and they are charged for the bandwidth they require (extortionate rates but that's neither here nor there).

They have an issue where they try to send an attachment in Outlook and it fails, I found that if I were to send a tiny attachment (50Kb) then it would send after a couple of minutes so it's able to send them but is incredibly slow. They have a 5Mb symmetrical connection and the testing was done with only one PC turned on so they have more than enough bandwidth for such small attachments. I have spoken to the building IT manager and their external IT company that set it all up but they are being less than helpful, they won't give me access to the switch to monitor stats or adjust the config and aren't willing to spend their own time on diagnosing either.

To rule out other possibilities we have changed one variable at a time:
Tried same computer, exchange account but using the rooms WiFi rather than wired - same issue
Tried different laptop on the same network (wired and WiFi) with same exchange account - same issue
Tried the laptop on their home network with the same exchange account - practically perfect

So the above should have ruled out the PC hardware, software and exchange account so we are back to where we began on the LAN. Seeing as we have enough bandwidth is it possible that any QoS may be affecting the data being sent by Outlook? The office had a problem with dropped packets on VoIP calls, I believe the other IT company worked setting up QoS, I have asked the client for a timeline of the issues.

I need to check but I believe if they use Outlook online they are able to send attachments fine, just thinking out loud here but if using Outlook Web Access would an attachment be uploaded over HTTP and then sent on from the mailserver but if from outlook it would use SMTP? I wonder if SMTP has somehow been deprioritised.

Any thoughts welcomed, I'm sure I will have to go back to the building management but the only was they will do anything is if I am assertive and I don' want to find out I overlooked something and it's not their fault!

Cheers