Why are alerts being triggered but emails are not getting sent?

[redhaducos]

curiously, scheduled reports are sending email with no issue.

in regards of the alerts , on which i have configured the actions: trigger and send by email.
i have this error message: in splunkd.log and python.log

sendemail:371 - Connection unexpectedly closed : [ Errno 104 ] Connection reset by peer when sending mail to : XXXX@XXX

[richgalloway]

Verify your email configuration (including credentials) are still correct. Then contact your mail admin to see what the logs say on his end.