mails bouncing from domain email server..help

[fireheart007]

hello all

i wanted to know about a problem with my work domain email address.
some mails tend to bounce to my domain email. shabz@unitedsports.com
why do some emails bounce?. also other collegues have said that if they dont open their outlook for a day .. the mails sent on that day bounces.... how is that possible? .. isnt the mail supposed to wait in my inbox untill i open it.. even if it is weeks later.... and one more point is that initially when i started using the domian email ... all hotmail emails bounced ... i was also unable to send to my hormail account... but then it became fine after a few days ... and now it seems some mail randomly bounce.. why ?... can i fix it?

another important point to note is when i open my outlook express.. it gives a message saying..
the server u r connected to is using a security certificate that could not be verified . a certificate chain processed but terminated ina root certificate which is not trusted by the trust provider . do u want to continue using this server?..
i say yes and use outlook. does this have anything to do with some bouncing mails... how can i fix this problem.. i want to send and recieve mails to/from everyone!
please help..

many thanks in advance