Hey,
Thank you for your reply.
Just checked the email Log and it is filled with Warnings and errors. Here is the dump of the first 50 Lines of the email log. The log was collected after one failed delivery for my test contact form submission.
Feb 7 18:16:47 ip-172-26-6-157 postfix/smtpd[238559]: connect from unknown[85.202.169.20]
Feb 7 18:16:49 ip-172-26-6-157 postfix/smtpd[238559]: warning: unknown[85.202.169.20]: SASL LOGIN authentication failed: Invalid authentication mechanism
Feb 7 18:16:49 ip-172-26-6-157 postfix/smtpd[238559]: disconnect from unknown[85.202.169.20] ehlo=1 auth=0/1 rset=1 quit=1 commands=3/4
Feb 7 18:20:09 ip-172-26-6-157 postfix/anvil[238561]: statistics: max connection rate 1/60s for (smtp:85.202.169.20) at Feb 7 18:16:47
Feb 7 18:20:09 ip-172-26-6-157 postfix/anvil[238561]: statistics: max connection count 1 for (smtp:85.202.169.20) at Feb 7 18:16:47
Feb 7 18:20:09 ip-172-26-6-157 postfix/anvil[238561]: statistics: max cache size 1 at Feb 7 18:16:47
Feb 7 18:22:31 ip-172-26-6-157 postfix/smtpd[238808]: connect from unknown[85.202.169.20]
Feb 7 18:22:31 ip-172-26-6-157 postfix/smtpd[238808]: warning: unknown[85.202.169.20]: SASL LOGIN authentication failed: Invalid authentication mechanism
Feb 7 18:22:32 ip-172-26-6-157 postfix/smtpd[238808]: disconnect from unknown[85.202.169.20] ehlo=1 auth=0/1 rset=1 quit=1 commands=3/4
Feb 7 18:22:33 ip-172-26-6-157 postfix/submission/smtpd[238816]: connect from unknown[45.146.164.178]
Feb 7 18:22:35 ip-172-26-6-157 postfix/submission/smtpd[238816]: SSL_accept error from unknown[45.146.164.178]: lost connection
Feb 7 18:22:35 ip-172-26-6-157 postfix/submission/smtpd[238816]: lost connection after STARTTLS from unknown[45.146.164.178]
Feb 7 18:22:35 ip-172-26-6-157 postfix/submission/smtpd[238816]: disconnect from unknown[45.146.164.178] ehlo=1 starttls=0/1 commands=1/2
Feb 7 18:25:55 ip-172-26-6-157 postfix/anvil[238810]: statistics: max connection rate 1/60s for (smtp:85.202.169.20) at Feb 7 18:22:31
Feb 7 18:25:55 ip-172-26-6-157 postfix/anvil[238810]: statistics: max connection count 1 for (smtp:85.202.169.20) at Feb 7 18:22:31
Feb 7 18:25:55 ip-172-26-6-157 postfix/anvil[238810]: statistics: max cache size 2 at Feb 7 18:22:33
Feb 7 18:28:02 ip-172-26-6-157 postfix/smtpd[239020]: connect from unknown[85.202.169.20]
Feb 7 18:28:04 ip-172-26-6-157 postfix/smtpd[239020]: warning: unknown[85.202.169.20]: SASL LOGIN authentication failed: Invalid authentication mechanism
Feb 7 18:28:04 ip-172-26-6-157 postfix/smtpd[239020]: disconnect from unknown[85.202.169.20] ehlo=1 auth=0/1 rset=1 quit=1 commands=3/4
Feb 7 18:28:36 ip-172-26-6-157 postfix/smtpd[239020]: connect from www12-azure.checktls.com[40.76.159.115]
Feb 7 18:28:37 ip-172-26-6-157 postfix/smtpd[239020]: disconnect from www12-azure.checktls.com[40.76.159.115] ehlo=2 starttls=1 mail=1 quit=1 commands=5
Feb 7 18:31:57 ip-172-26-6-157 postfix/anvil[239022]: statistics: max connection rate 1/60s for (smtp:85.202.169.20) at Feb 7 18:28:02
Feb 7 18:31:57 ip-172-26-6-157 postfix/anvil[239022]: statistics: max connection count 1 for (smtp:85.202.169.20) at Feb 7 18:28:02
Feb 7 18:31:57 ip-172-26-6-157 postfix/anvil[239022]: statistics: max cache size 2 at Feb 7 18:28:36
Feb 7 18:33:41 ip-172-26-6-157 postfix/smtpd[239249]: connect from unknown[85.202.169.20]
Feb 7 18:33:42 ip-172-26-6-157 postfix/smtpd[239249]: warning: unknown[85.202.169.20]: SASL LOGIN authentication failed: Invalid authentication mechanism
Feb 7 18:33:42 ip-172-26-6-157 postfix/smtpd[239249]: disconnect from unknown[85.202.169.20] ehlo=1 auth=0/1 rset=1 quit=1 commands=3/4
Feb 7 18:34:33 ip-172-26-6-157 dovecot[1372]: imap-login: Disconnected: Connection closed (no auth attempts in 1 secs): user=<>, rip=185.167.97.31, lip=172.26.6.157, TLS, session=<GwgS2nHXjN+5p2Ef>
Feb 7 18:35:31 ip-172-26-6-157 dovecot[1372]: imap-login: Disconnected: Connection closed (no auth attempts in 0 secs): user=<>, rip=45.146.164.178, lip=172.26.6.157, session=<PteK3XHXA/AtkqSy>
Feb 7 18:35:37 ip-172-26-6-157 dovecot[1372]: imap-login: Disconnected: Connection closed (no auth attempts in 1 secs): user=<>, rip=45.146.164.178, lip=172.26.6.157, session=<sP/d3XHXA/AtkqSy>
Feb 7 18:36:27 ip-172-26-6-157 dovecot[1372]: imap-login: Disconnected: Connection closed (no auth attempts in 0 secs): user=<>, rip=45.56.106.254, lip=172.26.6.157, session=<E+Pc4HHXSO4tOGr+>
Feb 7 18:37:02 ip-172-26-6-157 postfix/anvil[239251]: statistics: max connection rate 1/60s for (smtp:85.202.169.20) at Feb 7 18:33:41
Feb 7 18:37:02 ip-172-26-6-157 postfix/anvil[239251]: statistics: max connection count 1 for (smtp:85.202.169.20) at Feb 7 18:33:41
Feb 7 18:37:02 ip-172-26-6-157 postfix/anvil[239251]: statistics: max cache size 1 at Feb 7 18:33:41
Feb 7 18:37:14 ip-172-26-6-157 postfix/pickup[238101]: 7D118A2C8D: uid=5005 from=<tunnelt>
Feb 7 18:37:14 ip-172-26-6-157 postfix/cleanup[239454]: 7D118A2C8D: message-id=<9Vlv46BunKtBVmnevZkzpwspi61JdgI186IsNpwvc@tunnel2tech.com>
Feb 7 18:37:14 ip-172-26-6-157 opendkim[986]: 7D118A2C8D: DKIM-Signature field added (s=default, d=tunnel2tech.com)
Feb 7 18:37:14 ip-172-26-6-157 postfix/qmgr[3741]: 7D118A2C8D: from=<tunnelt@tunnel2tech.com>, size=1789, nrcpt=1 (queue active)
Feb 7 18:37:15 ip-172-26-6-157 postfix/smtp[239458]: 7D118A2C8D: to=<singhashwin27@gmail.com>, relay=gmail-smtp-in.l.google.com[2607:f8b0:400e:c08::1a]:25, delay=0.66, delays=0.14/0.02/0.15/0.36, dsn=5.7.1, status=bounced (host gmail-smtp-in.l.google.com[2607:f8b0:400e:c08::1a] said: 550-5.7.1 [2600:1f14:95:9100:f30f:9482:d5e4:71aa] Our system has detected that 550-5.7.1 this message does not meet IPv6 sending guidelines regarding PTR 550-5.7.1 records and authentication. Please review 550-5.7.1 https://support.google.com/mail/?p=IPv6AuthError for more information 550 5.7.1 . l186si10075607pgd.110 - gsmtp (in reply to end of DATA command))
Feb 7 18:37:15 ip-172-26-6-157 postfix/cleanup[239454]: 207B1A2C8E: message-id=<20220207183715.207B1A2C8E@tunnel2tech.com>
Feb 7 18:37:15 ip-172-26-6-157 postfix/bounce[239459]: 7D118A2C8D: sender non-delivery notification: 207B1A2C8E
Feb 7 18:37:15 ip-172-26-6-157 postfix/qmgr[3741]: 207B1A2C8E: from=<>, size=4909, nrcpt=1 (queue active)
Feb 7 18:37:15 ip-172-26-6-157 postfix/qmgr[3741]: 7D118A2C8D: removed
Feb 7 18:37:15 ip-172-26-6-157 postfix/pipe[239460]: 207B1A2C8E: to=<tunnelt@tunnel2tech.com>, relay=dovecot, delay=0.04, delays=0/0.01/0/0.03, dsn=5.1.1, status=bounced (user unknown)
Feb 7 18:37:15 ip-172-26-6-157 postfix/qmgr[3741]: 207B1A2C8E: removed
Feb 7 18:39:17 ip-172-26-6-157 postfix/smtpd[239687]: connect from unknown[85.202.169.20]
Feb 7 18:39:19 ip-172-26-6-157 postfix/smtpd[239687]: warning: unknown[85.202.169.20]: SASL LOGIN authentication failed: Invalid authentication mechanism
Feb 7 18:39:19 ip-172-26-6-157 postfix/smtpd[239687]: disconnect from unknown[85.202.169.20] ehlo=1 auth=0/1 rset=1 quit=1 commands=3/4
Feb 7 18:40:06 ip-172-26-6-157 dovecot[1372]: imap-login: Disconnected: Connection closed (no auth attempts in 0 secs): user=<>, rip=185.167.97.31, lip=172.26.6.157, TLS, session=<xKrx7XHX8IK5p2Ef>
Feb 7 18:40:18 ip-172-26-6-157 postfix/qmgr[3741]: 8F026A2C89: from=<lsadm@tunnel2tech.com>, size=911, nrcpt=1 (queue active)
Please help me understand this errors.