Allow configuring EHLO hostname in SMTP settings

[freezerxp]

Currently, it appears that the EHLO hostname is hardcoded in sendRawEmailWithSettings:

hostname := "localhost"
code, err = smtpConn.sendCommandMultiline(fmt.Sprintf("EHLO %s", hostname))

Because of this, as far as I understand, the SMTP client always identifies itself as:

EHLO localhost

which then appears in message headers as:

helo=localhost

In some SMTP server configurations this can affect how outgoing messages are processed. In our case DKIM signing on the SMTP server was not applied when the connection used EHLO localhost, while the same server signs messages correctly when a proper hostname is used.

It would be helpful to allow configuring the EHLO hostname in SMTP settings instead of hardcoding localhost.

For example, Listmonk exposes this as a configurable field in its SMTP settings, allowing administrators to specify the hostname used in the EHLO command.

[pierre-b]

Added in https://github.com/Notifuse/notifuse/releases/tag/v28.0 !

[linuxd3v]

I think this maybe breaking some things in certain cases.
For me authentication to notifuse stopped working and required manual fix #290