boldwater

Thank you, with these steps I was able to trace the issue to a recent update of the Rank Math SEO free plugin, and sent the developer a bug report.

Error is removed with the update, thanks.

Same error as above, everything else is up to date.

I am experiencing the same error message when sending a test email with wp 4.4.1 and latest Postman SMTP using SendGrid. Works on other sites, but this one has an old Theme and is on Godaddy hosting.

OS: Linux p3nlhg627.shr.prod.phx3.secureserver.net 2.6.32-604.30.3.lve1.3.63.el6.nfsfixes.x86_64 #1 SMP Mon Oct 26 11:56:49 MST 2015 x86_64
PHP: Linux 5.2.17 C
PHP Dependencies: iconv=Yes, spl_autoload=Yes, openssl=Yes, sockets=No, allow_url_fopen=Yes, mcrypt=Yes, zlib_encode=Yes
WordPress: 4.4.1 en_US UTF-8
WordPress Theme: Chameleon
WordPress Plugins: Gravity Forms, All In One SEO Pack, Geo Mashup, Get Custom Field Values, Google Analytics by Yoast, Postman SMTP, Royal Responsive Menu, Smooth Slider, WP Lightbox 2
WordPress wp_mail Filter(s): wp_staticize_emoji_for_email
Postman: 1.7.2
Postman Sender Domain (Envelope|Message): | MYDOMAINHERE.COM
Postman Prevent Message Sender Override (Email|Name): No | No
Postman Active Transport: SendGrid API (https://api.sendgrid.com:443)
Postman Active Transport Status (Ready|Connected): Yes | Yes
Postman Deliveries (Success|Fail): 10 | 0

Same error here upon updating. using WP 3.5.1

Thank you @gerygreen! The simple fix you posted made all exports work.