Some questions that might point you in the right direction, or provide us some information to help:
What version of osticket are you running? (is it working otherwise?)
What are your IMAP settings? (Are you using the ssl:// prefix to enable encryption on the server settings for the account?)
PHP and php_imap extension version?
Apache server version?
Does your VPS have an outbound firewall?
Can you telnet to the mail server either on SMTP or IMAP ports from the VPS?
What other troubleshooting steps did you take, apart from contacting your servers support?
Who is your VPS provider?
Who is your mail host? Did the account work before? Can you manually login now? (Make sure IMAP is enabled and the account not locked etc).