emails slow to recieve

I started using this plugin as Hotmail accounts would not receive emails from my website. This plugin fixed this problem. Great, thanks.

I have tested emails to the main email accounts via my website: Hotmail, Gmail, Yahoo, and they work fine. However, when I test emails to yahoo.com.au, they took at least 15 minutes to arrive in the inbox. Is there a way to fix this? If I send an email direct from my Gmail account to the yahoo.com.au account it arrives immediately.

Please see my diagnostics below, thanks:

OS: Linux p3nlwpweb136.shr.prod.phx3.secureserver.net 2.6.32-531.1.2.lve1.2.54.el6.nfsfixes.x86_64 #1 SMP Wed Apr 2 14:06:52 MST 2014 x86_64
HTTP User Agent: Mozilla/5.0 (Windows NT 6.3; WOW64; Trident/7.0; rv:11.0) like Gecko
Platform: PHP Linux 5.4.16 / WordPress 4.1.1 en_US
PHP Dependencies : iconv=Yes : spl_autoload=Yes : openssl=Yes : allow_url_fopen=Yes
WordPress Plugins : Contact Form 7 : Email Log : Floating Social Media Icon : Google Language Translator : Meks Easy Ads Widget : Postman Gmail API Extension : Postman SMTP : s2member Secure File Browser : s2Member Framework : Simple Google Analytics : Title Remover : Ultimate Coming Soon Page : wpDataTables
WordPress Theme: Twenty Fourteen
Postman Version: 1.5.12
Postman Sender: sh******os@gmail.com
Postman Transport URI: gmail_api:https:oauth2://1859****************************************************************.com:************************@www.googleapis.com:443
Postman Transport Status (Configured|Ready|Connected): Yes|Yes|Yes
Postman Deliveries (Success|Fail): 20|0
Postman Bind (Success|Fail): Yes|No
Postman Available Transports : Gmail API (1.0.1) : SMTP (1.5.12)
Postman LogLevel: 40000
Postman TCP Timeout (Connection|Read): 10|60

https://wordpress.org/plugins/postman-smtp/