rails sending bulk mails

Question

We are using Rails 2.3.8 version. In our application, we tend to send individual email to all users ( around 1500 users - Bulk email ). We are not using any services other than Actionmailer. Hence, we cannot use any service (like mailGun, sendGrid, mailchimp, etc) Also, we have our own SMTP server, through which all the mails are sent.

We have added our mailing functionality to be executed in two scenarios.

1. Rake Task with CRON JOB ( Auto mails to be sent at specific time)

2. Via controller ( Mails are sent when a button is clicked)

In both the cases, Initially, the mails are forwarded to the users without any issues. As the user count increases, suddenly the mail functionality stops and throws either of the two following errors.

1. 535 5.7.3 Authentication Unsuccessful
2. Execution expired

Could anyone please help me sort out the issue?

Is there any timeout value that has to be set for SMTP server connectivity? Or are there any restrictions (like number of mails sent per minute) ?

Thanks in Advance