Facing a delay in some emails (exchange server 2013)

Hello

I have an exchange server 2013 ,some senders sending emails to one or more of our contacts (under the same domain) , some one getting the email with delay & others didn't receive any thing  , I have disabled the antispam to ensure that it didn't blocking any emails or delaying it , also it didn't happened with all senders especially those using Hotmail or gmail .

if some body can help .

Thanks in advance