Google SMTP returning intermittent 421 4.7.0 errors

Context:

• Self-hosted on EC2
• Docker compose deployment

SMTP setup was completed using Google smtp-relay. Test emails were succesfully sent.

However, when adding new users, intermittently Google would close the connection prior to HELO with a 421 4.7.0 code. Therefore, these emails were never sent. Users couldn't log in.

Upon investigation:
Nodemailer uses os.hostname() for the SMTP EHLO greeting. In Docker containers, this returns the container ID (e.g., e508959023b2) instead of a valid FQDN. Google's SMTP relay rejects non-FQDN hostnames in EHLO, but inconsistently - causing intermittent failures.
Workaround:
Set hostname: your.domain.com in Docker Compose for the Rocket.Chat container.
Suggested Fix:
Expose nodemailer's name option in Rocket.Chat's SMTP settings (Admin → Email → SMTP). This option sets the EHLO hostname explicitly, independent of the container's system hostname.
References:
Nodemailer name option: https://nodemailer.com/smtp/ ("optional hostname of the client, used for identifying to the server")
Similar issue in other frameworks: https://github.com/loco-rs/loco/issues/1055

Start at the Admin → Email → SMTP settings entry point and inspect the Nodemailer SMTP integration described in the issue. Trace how SMTP options are stored and passed to the mailer, then verify that the configured client hostname is used for the EHLO greeting. Done means Docker deployments can set this value and Google SMTP relay no longer rejects the connection because of the container hostname.