Facing some weird issues, see below:

This is on the server the agent is running:

[root@xxx zabbix]# zabbix_agentd -t net.tcp.service[smtp]
net.tcp.service[smtp]                         [u|1]
[root@xxx zabbix]#

This is on the zabbix server:

[root@xxxx zabbix]# zabbix_get -s xxx.xx.xxx.x -k net.tcp.service[smtp]
0

All other checks such as HTTP, POP, IMAP work fine. Check for all connectivity issues, nothing found.