Mikrotik routers, like most others, do not directly support the use of hostnames instead of IP addresses.

However, we can use a simple script to resolve a hostname to an address-list, even if the hostname points to multiple IP addresses.

This script is intended to resolve multiple hostnames in the source= line.

/system script
add comment="Resolve DNS Hostname to Address List" name=resolve_delivery.antispamcloud.com owner=admin \
    policy=ftp,reboot,read,write,policy,test,password,sniff,sensitive,romon \
    source=":local hosts {\"delivery.antispamcloud.com\"}\r\
    \n:foreach k,v in=\$hosts do={\r\
    \n  :log info \"Doing \$v\"\r\
    \n  :local listname \$v\r\
    \n  :resolve \$v\r\
    \n  :local iscname [/ip dns cache all find where name=\$v and type=\"CNAME\"]\r\
    \n  :if (\$iscname != \"\") do={\r\
    \n    :local newname [/ip dns cache all get \$iscname data]\r\
    \n    :log info \"\$v is CNAME to \$newname\"\r\
    \n    :set v \$newname\r\
    \n  }\r\
    \n  :resolve \$v\r\
    \n  /ip firewall address-list remove [/ip firewall address-list find where list=\$listname]\r\
    \n  :foreach i in=[/ip dns cache all find where name=\$v and type=\"A\"] do={\r\
    \n    :local ipaddr [/ip dns cache all get \$i data]\r\
    \n    /ip firewall address-list add list=\$listname address=\$ipaddr comment=\$v\r\
    \n    :log info \"IP address: \$ipaddr\"\r\
    \n  }\r\
    \n  /ipv6 firewall address-list remove [/ipv6 firewall address-list find where list=\$listname]\r\
    \n  :foreach i in=[/ip dns cache all find where name=\$v and type=\"AAAA\"] do={\r\
    \n    :local ipaddr [/ip dns cache all get \$i data]\r\
    \n    /ipv6 firewall address-list add list=\$listname address=\$ipaddr comment=\$v\r\
    \n    :log info \"IPv6 address: \$ipaddr\"\r\
    \n  }\r\
    \n}\r\
    \n:log info \"end\""