Look at the links in this post if they can be useful. Also try taking the test in ssh from vcenter.
Are there any firewalls between the vcenter network and the client where you receive the mail (port 25)?
Can you check the network connections (TCP/UDP) between the vCenter server and your Mail server? Maybe something prevent your E-Mail operations like firewall, or maybe it's because of a wrong/invalid settings for sending E-Maill (in alert definition) .... however please check the communication between the VC and MX, via running the tcpdump on the vcenter shell like these:
tcpdump -i eth0 tcp port 25 (SMTP default port, if you used)
tcpdump -i eth0 dst 188.8.131.52 (IP address of the Mail Server)Please mark my comment as the Correct Answer if this solution resolved your problem