smtp send fails with error Net::OpenTimeout: execution expired

Summary

New install of GitLab-CE using Omnibus for Debian 8 on Digital Ocean 4GB droplet will not send emails via SMTP (using GMail as provider) and fails with Net::OpenTimeout: execution expired

Steps to reproduce

  1. Install GitLab-CE using Omnibus: https://about.gitlab.com/downloads/#debian8

  2. Configure SMTP using settings for GMail: https://gitlab.com/gitlab-org/omnibus-gitlab/blob/master/doc/settings/smtp.md

  3. Add users

  4. Create test project

  5. Raise issue

  6. Impersonate users and add comments

  7. Review Sidekiq logs

Results of GitLab application Check

Checking GitLab Shell ...

GitLab Shell version >= 3.6.1 ? ... OK (3.6.1)
Repo base directory exists?
default... yes
Repo storage directories are symlinks?
default... no
Repo paths owned by git:git?
default... yes
Repo paths access is drwxrws---?
default... yes
hooks directories in repos are links: ... 
2/1 ... ok
3/2 ... ok
3/3 ... ok
3/4 ... ok
Running /opt/gitlab/embedded/service/gitlab-shell/bin/check
Check GitLab API access: OK
Access to /var/opt/gitlab/.ssh/authorized_keys: OK
Send ping to redis server: OK
gitlab-shell self-check successful

Checking GitLab Shell ... Finished

Checking Sidekiq ...

Running? ... yes
Number of Sidekiq processes ... 1

Checking Sidekiq ... Finished

Checking Reply by email ...

Reply by email is disabled in config/gitlab.yml

Checking Reply by email ... Finished

Checking LDAP ...

LDAP is disabled in config/gitlab.yml

Checking LDAP ... Finished

Checking GitLab ...

Git configured with autocrlf=input? ... yes
Database config exists? ... yes
All migrations up? ... yes
Database contains orphaned GroupMembers? ... no
GitLab config exists? ... yes
GitLab config outdated? ... no
Log directory writable? ... yes
Tmp directory writable? ... yes
Uploads directory setup correctly? ... yes
Init script exists? ... skipped (omnibus-gitlab has no init script)
Init script up-to-date? ... skipped (omnibus-gitlab has no init script)
projects have namespace: ... 
2/1 ... yes
3/2 ... yes
3/3 ... yes
3/4 ... yes
Redis version >= 2.8.0? ... yes
Ruby version >= 2.1.0 ? ... yes (2.3.1)
Your git bin path is "/opt/gitlab/embedded/bin/git"
Git version >= 2.7.3 ? ... yes (2.7.4)
Active users: 9

Checking GitLab ... Finished

Results of GitLab environment info

System information
System:		Debian 8.6
Current User:	git
Using RVM:	no
Ruby Version:	2.3.1p112
Gem Version:	2.6.6
Bundler Version:1.13.3
Rake Version:	10.5.0
Sidekiq Version:4.1.4

GitLab information
Version:	8.12.7
Revision:	7429b21
Directory:	/opt/gitlab/embedded/service/gitlab-rails
DB Adapter:	postgresql
URL:		https://gitlab.topdawgtech.net
HTTP Clone URL:	https://gitlab.topdawgtech.net/some-group/some-project.git
SSH Clone URL:	git@gitlab.topdawgtech.net:some-group/some-project.git
Using LDAP:	no
Using Omniauth:	no

GitLab Shell
Version:	3.6.1
Repository storage paths:
- default: 	/var/opt/gitlab/git-data/repositories
Hooks:		/opt/gitlab/embedded/service/gitlab-shell/hooks/
Git:		/opt/gitlab/embedded/bin/git

Other Info

This installation is to replace an existing GitLab server that is working as expected.

I copied the working SMTP settings in /etc/gitlab/gitlab.rb from old-server into new-server and confirmed that it's still not working