… But if you can log into the box with ssh and run tail -f /var/log/mail.log and copy here the output that comes up at the moment Outlook tries to send an email, that could help.
Oct 18 14:27:59 ip-10-138-34-11 postfix/smtpd[30355]: warning: hostname abts-tn-dynamic-255.114.164.122.airtelbroadband.in does not resolve to address 122.164.114.255: Name or service not known
Oct 18 14:27:59 ip-10-138-34-11 postfix/smtpd[30355]: connect from unknown[122.164.114.255]
Oct 18 14:28:00 ip-10-138-34-11 postfix/smtpd[30355]: lost connection after UNKNOWN from unknown[122.164.114.255]
Oct 18 14:28:00 ip-10-138-34-11 postfix/smtpd[30355]: disconnect from unknown[122.164.114.255]
Oct 18 14:28:00 ip-10-138-34-11 postfix/smtpd[30355]: warning: hostname abts-tn-dynamic-255.114.164.122.airtelbroadband.in does not resolve to address 122.164.114.255: Name or service not known
Oct 18 14:28:00 ip-10-138-34-11 postfix/smtpd[30355]: connect from unknown[122.164.114.255]
Oct 18 14:28:00 ip-10-138-34-11 postfix/smtpd[30355]: lost connection after STARTTLS from unknown[122.164.114.255]
Oct 18 14:28:00 ip-10-138-34-11 postfix/smtpd[30355]: disconnect from unknown[122.164.114.255]
Oct 18 14:28:00 ip-10-138-34-11 postfix/smtpd[30355]: warning: hostname abts-tn-dynamic-255.114.164.122.airtelbroadband.in does not resolve to address 122.164.114.255: Name or service not known
Oct 18 14:28:00 ip-10-138-34-11 postfix/smtpd[30355]: connect from unknown[122.164.114.255]
Oct 18 14:28:01 ip-10-138-34-11 postfix/smtpd[30355]: lost connection after EHLO from unknown[122.164.114.255]
Oct 18 14:28:01 ip-10-138-34-11 postfix/smtpd[30355]: disconnect from unknown[122.164.114.255]
Oct 18 14:28:01 ip-10-138-34-11 postfix/submission/smtpd[30357]: warning: hostname abts-tn-dynamic-255.114.164.122.airtelbroadband.in does not resolve to address 122.164.114.255: Name or service not known
Oct 18 14:28:01 ip-10-138-34-11 postfix/submission/smtpd[30357]: connect from unknown[122.164.114.255]
Oct 18 14:28:01 ip-10-138-34-11 postfix/submission/smtpd[30357]: lost connection after UNKNOWN from unknown[122.164.114.255]
Oct 18 14:28:01 ip-10-138-34-11 postfix/submission/smtpd[30357]: disconnect from unknown[122.164.114.255]
Oct 18 14:28:01 ip-10-138-34-11 postfix/submission/smtpd[30357]: warning: hostname abts-tn-dynamic-255.114.164.122.airtelbroadband.in does not resolve to address 122.164.114.255: Name or service not known
That didn’t shed any light unfortunately. Looks like Outlook is disconnecting on its own, even though you see an error message. So I’m not sure. Until I get a chance to try Outlook myself, I don’t have any suggestions. (Except to try a different mail client, like Thunderbird.)
When using Thunderbird to connect, you should set all the parameters by hand. Don’t let it automatically find out, as it didn’t work here. By settings SSL, TLS, ports, etc. manually it was possible to connect.