I'm having a similar issue with contact form 7 and postman. I can't send email from contact form 7 to Zoho mail. The thing is that postman says that the email is being sent, but I can't see it in my inbox.
This is what I'm getting in Diagnostic test:
OS: Linux a2plcpnl0263.prod.iad2.secureserver.net 2.6.32-531.29.2.lve1.3.11.10.el6.x86_64 #1 SMP Fri Jun 12 15:09:02 EDT 2015 x86_64
HTTP User Agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10_9_5) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/44.0.2403.157 Safari/537.36
Platform: PHP Linux 5.4.43 / WordPress 4.3 es_ES
PHP Dependencies: iconv=Yes, spl_autoload=Yes, openssl=Yes, sockets=Yes, allow_url_fopen=Yes, mcrypt=Yes
WordPress Theme: AutoImage
WordPress Plugins: Akismet, Contact Form 7, Limit Login Attempts, Log Viewer, Master Slider, Postman SMTP, Go Daddy Quick Setup, Regenerate Thumbnails, WordPress Importer, Clef
Postman Version: 1.6.23
Postman Sender Domain (Envelope|Message): elmejorcarro.com|elmejorcarro.com
Postman Prevent Message Sender Override (Email|Name): No|No
Postman Transport URI: smtp:ssl:plain://smtp.zoho.com:25
Postman Transport Status (Configured|Ready|Connected): Yes|Yes|Yes
Postman Deliveries (Success|Fail): 2|3
Postman Bind (Success|Fail|Path): Yes|No|/home/elmejorcarro/public_html/wp-content/plugins/postman-smtp/Postman/PostmanWpMailBinder.php
Postman TCP Timeout (Connection|Read): 10|60
Postman Email Log (Enabled|Limit|Transcript Size): Yes|10|128
Postman Run Mode: production
Postman PHP LogLevel: 40000
Postman Stealth Mode: No
Postman File Locking (Enabled|Temp Dir): Yes|/tmp