Sendmail stopped working at some point after Feb 1, 2020

Has been working for years on my EC2 t2micro instance with Linux.

No matter where I send email, I get back this in maillog:
dsn=2.0.0, stat=Sent (01TFcr4c003013 Message accepted for delivery)
about 5 min go by, and then:
...dsn=4.0.0, stat=Deferred: Connection timed out with <whatever server name>.

Tried rebooting the instance. No effect.

Tried to configure sendmail.mc with port 143 explicitly (followed by make and sendmail restart). Modified the following line:

DAEMON_OPTIONS(Port=smtp,Addr=127.0.0.1, Name=MTA')dnl** to **DAEMON_OPTIONS(Port=143,Addr=127.0.0.1, Name=MTA')dnl
After that, getting another error:
dsn=4.0.0, stat=Deferred: Connection refused by [127.0.0.1]

Not sure what else to do. A little help please.
Thanks.