Visa advice needed , I will be applying for a visa from the UK (London Embassy) using - Thai E visa official website , my children are half Thai half English , their mother and I are divorced , one child is registered in custody to me , what are my visa options (LTR ?? Multiple entry) would I qualify for this ? ,,, All help much appreciated thanks in advance ?

I'm helping my mother in law apply for a tourist visa from london. They emailed requesting us to resend the flight and hotel booking but when we attach the requested docs and reply to the email we get it bounced back saying,

"Address not found

Your message wasn't delivered to thaievisa@mfa.go.th because the address couldn't be found, or is unable to receive mail."

Anyone have a similar issue and any suggestions?