Apple Mail Sending issues

I have a Mac Book Pro.


Model Name: MacBook Pro
Model Identifier: MacBookPro6,2
Processor Name: Intel Core i7
Processor Speed: 2.66 GHz
Number Of Processors: 1
Total Number Of Cores: 2
L2 Cache (per core): 256 KB
L3 Cache: 4 MB
Memory: 4 GB
Processor Interconnect Speed: 4.8 GT/s
Boot ROM Version: MBP61.0057.B0C
SMC Version (system): 1.58f15
xxxxx
System Version: Mac OS X 10.6.8 (10K549)
Kernel Version: Darwin 10.8.0
Boot Volume: Macintosh HD
Boot Mode: Normal
Computer Name: Finn's MacPro
User Name: Finn Davey (finndavey)
Secure Virtual Memory: Enabled
64-bit Kernel and Extensions: No


We asked a third party to register our domain name and set up our emails. Our domain name is wajibu.com and as the administrator I can manage the emails on a Parallels Plesk Panle 9.5.2 for Microsoft Windows panel.
We use all Mac computers and use Apple Mail as our message program. After setting up mail it has been easy to receive mail into the AMail inbox, but it is not possible to send mail. I have deleted and input new accounts several times. I have approached the party that registered and set up our addresses, but still I have the problem. I believe that there is some conflict between our Apple Male and the Plesk Panel but I can not find out how to fix it or sort it. I have getting very frustrated. Maybe someone out there can help.