You can connect your SMTP server with telnet
Then test with EHLO command
>telnet
smtp-relay.gmail.com 587
220
smtp-relay.gmail.com ESMTP 41be03b00d2f7-af8a27f3c36sm273271a12.7 - gsmtp
EHLO
smtp-relay.gmail.comEHLO
smtp-relay.gmail.com250-smtp-relay.gmail.com at your service, [2403:6200:8851:17c0:c016:5e32:7a4b:8d2f]
250-SIZE 157286400
250-8BITMIME
250-STARTTLS
250-ENHANCEDSTATUSCODES
250-PIPELINING
250-CHUNKING
250 SMTPUTF8
250-STARTTLS --> set your email server option to STARTTLS
Otherwise, check your SMTP server if it needs authentication