[smtp-check]: Some MTA do reject client connections unless the local name looks like an fqdn

[znerol]

The golang net/smtp package defaults to localhost for the EHLO local name unless specified otherwise using the c.Hello func.

Some MTA do reject client connections unless the local name at least looks like an fqdn. Extract from a sample run:

2023/07/14 16:08:19 === MX: 10  [...].
2023/07/14 16:08:19 SPF pass for [...]: matched ip
2023/07/14 16:08:24 TLS error: 550 Server name localhost is not a FQDN.
Please contact your ISP or system adminstrator.
2023/07/14 16:08:24 Allowed by policy

[Alberto Bertogli]

Thanks a lot for sending this!

It looks good to me. I merged it to the next branch, with some minor amends:

• Minor edits to the commit message (just editorial, e.g. "MTA" -> "MTAs").
• Ran go fmt.
• Renamed flag --local_name -> --localname for consistency with most other flags.

Can you please check ad0dbb9 and confirm you're okay with this?

Thanks again!

[znerol]

Perfect! Thanks a lot.