The following notes have some things to check for. Do you have multiple instances sharing the same mailbox?
Configuring IMAP4 For Workflow Mailer Errors: Unable to connect to the mail account. Check the host, user name and password (Doc ID 1081089.1)
Mailer cannot Connect to IMAP Server on Oracle Applications R12.1.1 (Doc ID 1300542.1)
Mailer SMTP Authentication Fails with Shared Mail Box Account (Doc ID 1946771.1)
How to Troubleshoot 'Validation failed' Errors when Starting the Mailer (Doc ID 463354.1)
Is your mail service in the cloud? Office365? If so, have you consulted your firewall person?