SMTP Authentication login

Operating system: Centos
OS version: 8

I’m setting up virtualmin for the first time. I have 3 domains and I’m the only user. I have several email aliases and most forward to my gmail account. I have one account that gmail accesses via pop3.

For each alias, in gmail, I must setup the SMTP to point to my SMTP server. There is a place to indicate the authentication credentials.

Does it matter if I use the same credentials for all the aliases? or should I use the credential associated with each domain account?

Also, I’m trying my best to make sure everything is secure.

I’m trying to determine the minimum number of ports I need the firewall the let through. I understand I must have port 25 open to receive email. 995 for POP3 TLS/SSL. For outgoing SMTP, do I need 587 for TLS or will it work over 25?

I’d be interested in what else I can do to protect my email (and apache) server from attacks.

An alias does not have an account, and thus has no credentials. You login with whatever account the alias directs mail to. If you want different credentials, you need different users.

Edit: Or do you mean a “domain alias with own mail”? In which case, the mail users would have their own credentials, since they are different users from the primary domain.

I mean an email alias.

I changed the domain names for privacy but here is an example of what I have.

Server hostname: server.first.com
IP RDNS: first.com

first.com domain
email alias foo@first.com forward to my@gmail.com
email alias bar@first.com forward to my@gmail.com
actual email box eng3@first.com

second.com domain
email alias foo@second.com forward to my@gmail.com

However, I initially created an ‘eng3’ account on the server as my typical non-priv account login. There are several accounts ‘eng3’, (‘eng3@first.com’, ‘eng3-first.com’), ‘first’, ‘second’

If I email eng3@first.com, it goes to eng3-first.com@first.com.

In gmail, I add foo@first.com, bar@second.com and foo@second.com to the “send mail as” section where I configure SMTP settings for each email address. There I need to provide the SMTP authentication credentials. Which login do I provide? do I give the one that corresponds to the email account or domain? can I just give the same login for all of them?