That’s an issue on the receiving end – and the reasons could be varied. For the most part, the reasons have nothing to do with the plugin itself (and may or may not have anything to do with WP – it depends).
I would recommend reviewing the FAQ here: Email troubleshooting (rocketgeek.com) Especially the part about setting up SMTP. If the rejection is based on formatting, then using an authenticated SMTP account will help solve that issue. (And regardless of the ultimate issue, you should do this anyway)
It could be something as simple as the headers of the email and/or that your sending info is not the same as the domain (common on shared hosting). These can be solved – most easily by using an SMTP account as described in the FAQ link above.
The other possible reasons have to do with spam filtering (and rejection of potential spam messages), and none of what follows has anything to do with the plugin or WordPress. Gmail (and others such as hotmail.com and other MS derivatives like outlook.com/live.com) require that you have SPF, DKIM, and/or DMARC configured for your email domain. In most cases, you only need one or the other, but you’ll need one (SPF is the easiest to set up). Yahoo is less strict on these which could explain why you’re having issues with gmail and not yahoo.com
Other issues could be that your domain and/or sending IP are on a ban-list, or that you have some other configuration issue.
There is a good tool for checking your MX at the link below. You can check your SPF record, MX config, and see if you are on a ban list: https://mxtoolbox.com/SuperTool.aspx
Please note that most (or all, actually) of this has nothing to do with the plugin. Email receiving has to do with what happens on the receiving mail server. To solve those issues, you have to have some experience with email and proper domain/DNS zone configuration.
okay thanks for the info!
