How can i improve mail box users send and receive latency in ms exchange 2013

our ms Exchange server has been installed 6 months ago and has been working fine until some few days ago. Some users are able to send emails out and receive but the emails are received by the receiver after 2 hours some after 1 hour some after several minutes. Im new to ms exchange so i don't really know what to do.

Some users that rarely use emails are able to send and receive normally, but those users that use email every minute they are sending emails out but no longer can receive email coming in anymore.

I have seen that the mailbox quota set is only 2GB. Im assuming those actives users have reached their quotas because most of their quotas are 85% - 95%  full.

What can i do ??
Henry WouAsked:
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8046586Commented:
Check the server activity and CPU performance.
Do you have antivirus for exchange? Check the antivirus logs.
What you are explaining you either have very slow internet connection from time to time, or you are under attack. You might be sending or receiving SPAM e-mails. Scan the incoming and outgoing queue. Check if some of those e-mails are sent to random addresses (ex. Yahoo, Gmail, live, Hotmail etc.). You might be receiving a lot of spam, and your server is distributing e-mails slow, and antivirus works like crazy.
Another thing to check is relaying e-mails, do you have any open relay?
If server CPU is working on 100% than the quickest troubleshooting is it related to the sending/receiving issue just pool the WAN cable of it. If the CPU drop below 40% than definitely you have extreme sending/receiving.
A long time ago I had the similar issue back in 2003. After 8 hours troubleshooting, we found that 3-5 users sent a video clip as an attachment to more than 200 external addresses. You might have over-excited users who are sharing their new year eve clips over the e-mail.
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Dr. KlahnPrincipal Software EngineerCommented:
Cut the mailbox quota to 1 GB, which is still too big but is more than sufficient for email use.  Excessively large mailbox quotas encourage people to use email as a filing system, which it is not.  Tell users to keep their mailboxes well below that limit or they won't be able to receive incoming email.  Don't make exceptions for people who "need a bigger mailbox to keep stuff in."  Email is for passing messages and the occasional file, not for "keeping stuff in."

Monthly, prune all email over (one year, 9 months, whatever) old.  This encourages users to get anything that is truly important out of the email system.
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Sajid Shaik MSr. System AdminCommented:
go t oroganization configuration-mailbox-DatabaseManagement-(select Databse properties) -limits check the mailbox limits.

if it's only on few users so check the machines  A/v logs

mail flow on exchange -do the following

emc-Toolbox- queviewer..
emc-Toolbox- mailflow troubleshooter..

all the best
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Todd NelsonSystems EngineerCommented:
Sufficient info provided for resolution.
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