e-mail not working vcenter 6.5

After upgrading to vcenter appliance to 6.5, it stopped sending alarm email notifications.

Looking in the the vpxd.log, I noticed the following:

017-01-17T10:32:42.464Z info vpxd[7F4B661C3700] [Originator@6876 sub=MoOptionMgr opID=VcConfigWizard-apply-60719-ngc-74] Setting SMTP relay server to [mailgw.cti.gr]:25

2017-01-17T10:32:42.523Z error vpxd[7F4B661C3700] [Originator@6876 sub=MoOptionMgr opID=VcConfigWizard-apply-60719-ngc-74] Cannot set SMTP relay server:Error code configuring sendmail: 2

Any ideas?

Thanks in advance,

Jason