Hi ,
Thank you so much for providing detailed information about your job. It instantly grabbed my attention and aligns perfectly with my experience as a Mail Flow Eng.
As you can view on my profile , I've completed numerous jobs with 5-star review , including overly positive client feedback that showcase the quality of my work and relate directly to your post are attached to this proposal for you as well.
I can check this error you got but most probably the problem is from the destination server ,To dig more :
1- May I ask you please to send email to different email provider ?
2- May I ask you please to give me your domain name ?
Best Regards