Postman: wp_mail has been declared by another plugin or them

I am getting this message on one of my sites:
“Postman: wp_mail has been declared by another plugin or theme, so you won’t be able to use Postman until the conflict is resolved.”

I deactivated a number of my plugins, and re-checked the diagnostic with this result:

OS: Linux box861.bluehost.com 3.12.50-39.ELK6.x86_64 #1 SMP Mon Nov 2 03:10:26 CST 2015 x86_64
PHP: Linux 5.6.17 C
PHP Dependencies: iconv=Yes, spl_autoload=Yes, openssl=Yes, sockets=Yes, allow_url_fopen=Yes, mcrypt=Yes, zlib_encode=Yes
WordPress: 4.5.3 en_US UTF-8
WordPress Theme: Graphene
WordPress Plugins: TDO Mini Forms, AdSense Integrator, Akismet, Contact Form 7, Contextual Related Posts, Custom fields shortcode, Gallery Manager Lite, Graphene Shortcodes, Jetpack by WordPress.com, List category posts, List Pages Shortcode, Official StatCounter Plugin, Post Type Switcher, Postman SMTP, Really Simple CAPTCHA, Optimize Database after Deleting Revisions, SI CAPTCHA Anti-Spam, Smart Ads, Theme My Login, Top 10, T&P Gallery Slider, UpdraftPlus – Backup/Restore, WP Easy Paypal Payment Accept, WP Simple Paypal Shopping cart, Wp-Insert, WP Mobile Edition, WP Super Cache, WP125
WordPress wp_mail Owner: …./wp-includes/pluggable.php
WordPress wp_mail Filter(s): wp_staticize_emoji_for_email
WordPress phpmailer_init Action(s): wpcf7_phpmailer_init
Postman: 1.7.2
Postman Sender Domain (Envelope|Message): gmail.com | gmail.com
Postman Prevent Message Sender Override (Email|Name): No | No
Postman Active Transport: Gmail API (https:oauth2://www.googleapis.com:443)
Postman Active Transport Status (Ready|Connected): Yes | Yes
Postman Deliveries (Success|Fail): 0 | 0

The plugins that are still active (as above) are also in use on my other sites, where PostmanSMTP is working OK.

The following Jetpack plugins are also active:
Carousel
Custom CSS
JSON API
Omnisearch
Protect
Sharing
Single Sign On
Site Stats
Site Verification
Sitemaps
Widget Visibility

Any ideas for me to fix this?

https://www.ads-software.com/plugins/postman-smtp/