Hi @asheroto ,
Thanks for the additional information. I checked their plugin and it looks like they’re loading the pluggable file directly.
It isn’t recommended to require that file inside a plugin. All functions inside the pluggable.php file are overridable. If a plugin requires it (like their plugin), other plugins won’t be able to override the functions inside it (including wp_mail).
I would recommend asking them if it’s possible to not include the pluggable.php file directly in their plugin.
“Is it just the way that a plugin is using the wp_mail function? How do other plugins email without issue while some fail? You mentioned it being overridden?”
— I couldn’t say for sure how other plugins send their emails, but it’s possible that they are using their own function to send emails and they bypass the wp_mail function.
We also encountered an issue before where if a plugin is loaded first before WP Offload SES, and they are also overriding the wp_mail function, then their plugin would get to use wp_mail. This would mean WP Offload SES won’t work.
Let us know if you have any other questions.