Email send issue for numbers of email

[cttech]

The script is working fine for few emails but when I am trying to send more than 30-40 emails it's failed to send emails. Some error message from my log file is given below.

09:55:27 > Mailing Complete.
Sending failed: SMTP Error: Could not connect to SMTP host.
09:55:14 > Started Mailing MTA. Spawn #4.
09:55:14 > Runtime (80 seconds) reached, respawning.
Sending failed: SMTP Error: Could not connect to SMTP host.
Sending failed: SMTP Error: Could not connect to SMTP host.
Sending failed: SMTP Error: Could not connect to SMTP host.
Sending failed: SMTP Error: Could not connect to SMTP host.
Sending failed: SMTP Error: Could not connect to SMTP host.
Sending failed: SMTP Error: Could not connect to SMTP host.

10:59:43 > Mailing Complete.
Sending failed: The following From address failed: [email protected]
SMTP server error: sender already given