$ host –t MX va.
va mail is handled by 10 lists.vatican.va.
va mail is handled by 50 proxy2.urbe.it.
va mail is handled by 20 paul.vatican.va.
va mail is handled by 90 john.vatican.va.
$

1. The subshell in your one-liner is entirely unnecessary:

for domain in $(grep -v '^#' tlds-alpha-by-domain.txt) ; do host -t A "${domain}."; done | grep -v 'has no A record'

2. The backticks can be avoided:

grep -v '^#' tlds-alpha-by-domain.txt | while read domain ; do host -t A "${domain}."; done | grep -v 'has no A record'

3. This loop can be nicely replaced by a use of xargs -i:

grep -v '^#' tlds-alpha-by-domain.txt | xargs -i host -t A -W 1 {}. | grep -v 'has no A record'