hi, after many ours debugging here some help for others stumbling on the same.
Mymail in cinjunction with amazon ses works great but for internal wp forms like form contcat form 7 its not working stable. To many problems. sometimes it workjs somettimes not.
My hoster advices me to install a wp smtp handler and that did the job. see https://cool-tricks.net/contact-form-7-configuration/ for config of https://wordpress.org/plugins/wp-mail-smtp/
I still use the option ‘use mymail template only for wp mail’ and in that combination it works great.