Can you share sample oml of timer?
No, those types of errors won't hit any exception handlers.
The part of the email sending to the smtp server is asynchronous (and even does retries when fails).
If your use case is to know if there are any emails that failed you can reference the Email and Email_Status from the System reference and check there.It's more reliable that way than to check the logs.
I'm having the same errors as Linguo showed in the first message of this post. But in my case, i'm using EmailSend action from eMailServices eSpace. The 550 error refers to a non existing email but i'm sure this email exists. Can it be something related to the DNS that is configured from the server side?