Our add-on cannot access the SMTP port in case you are using your own SMTP server protected by a firewall. In order to work around this, you need to allow certain IP addresses on your firewall.
Current IP addresses
Email This Issue is currently running on Amazon EC2 instances. It is currently using IP address 52.27.95.185 to connect to IMAP and SMTP servers. After 15 May 2020 we are going to add two more IP addresses to increase our network redundancy, so if you are using a firewall to filter incoming connections to your IMAP or SMTP servers, then please add these two new addresses to your firewall configurations (in addition to the original 52.27.95.185):
54.188.78.95
52.12.208.63
In conclusion, please allow inbound connections from these IP addresses:
52.27.95.185/32
54.188.78.95/32
52.12.208.63/32