Verification mail sending issue

You must ensure that the outgoing emails meet the requirements. The domain must have an SPF record and DKIM set, which is probably the case otherwise Gmail would reject them. Another criterion may be that the email is not sent authenticated from WordPress. Plugins such as https://www.ads-software.com/plugins/wp-mail-smtp/ can help here. There you have to create an account on your email server through which all outgoing emails are sent.

If this still doesn’t work, you will need to ask the email service provider of the receiving server if they can tell you anything about this.