"Tl;dr Use port 587 if you can, 465 if you can’t, 25 if you must.

Port 587 is technically correct, the best kind of correct. However, many ESPs have adopted implicit TLS on port 465. While you can send email over port 25 and 2525, it’s much more secure to have the messages encrypted. This makes port 587 the preferred option for sending, with port 465 as a close second."

• https://sendgrid.com/blog/whats-the-difference-between-ports-465-and-587/

"This is the default mail submission port. When users submit an email to be routed by a proper mail server, this is the one that will provide best results.
Port 587, coupled with TLS encryption, ensures that email is submitted securely and following the guidelines set out by the IETF. All Mailgun customers should consider using port 587 as their default SMTP port unless you're explicitly blocked by your upstream network or hosting provider."

• https://www.mailgun.com/blog/email/which-smtp-port-understanding-ports-25-465-587/

"Setting secure to false does not mean that you would not use an encrypted connection. Most SMTP servers allow connection upgrade via STARTTLS command but to use this you have to connect using plaintext first"

"secure – if true the connection will use TLS when connecting to server. If false (the default) then TLS is used if server supports the STARTTLS extension. In most cases set this value to true if you are connecting to port 465. For port 587 or 25 keep it false"

"requireTLS – if this is true and secure is false then Nodemailer tries to use STARTTLS even if the server does not advertise support for it. If the connection can not be encrypted then message is not sent"

Using @squadcal.org resulted in the following:

ErrorCode: '400', Message: 'The 'From' address you supplied (no-reply@squadcal.org) is not a Sender Signature on your account. Please add and confirm this address in order to be able to use it in the 'From' field of your messages.'.

Figured @comm.app makes more sense anyways.