what is the significance of "mail" in dns mail.domain.com?

what is the significance of "mail" in dns mail.domain.com?
I am trying to understand this and find it a little confusing. take the following server situation...
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a shared webhosting/mail debian 9 server running apache and postfix+dovecot

the server hostname = server1
hostname fqdn = server1.hostdomain.com
External Public Static IP address = 12.34.56.78

client = clientdomain.com

All dns is controlled externally.
1. server1.hostdomain.com is resolved via A record at its registrar (using free dns offered by registrar)
2. client websites are pointed at the server via A record at their registrar


Questions
1. What is the significance of "mail" in mail.domain.com often seen in dns records/tutorials? Is this supposed to be the hostname or is it a dns default for email? What if the mail server hostname is "server1" (as in my example)
OR
is mail.domain.com simply used by email clients as a default in order to setup accounts? so a dns record mail.domain.com is always required for email clients to use? (if so, should mail.domain.com point to an IP address or the mail server hostname.fqdn? (ie server1.hostdomain.com)

2. Using my example above, exactly what dns records should be shown for email to work for clients on this system?
just to update these are the email dns records i am using at present for clientdomain.com...(I havent included TTLS)
A record 12.34.56.78
MX record server1.hostdomain.com 10
TXT record v=spf mx a 12.34.56.78 a : server1.hostdomain.com ~all

does that all seem good?

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One thing that has caused me to raise this issue is I cannot get my Samsung Galaxy Note 9 Microsoft Outlook App to connect to any email accounts for this domain. What does the Outlook app need in dns to achieve this?

I am able to setup email accounts on gmail android app, although i get an SSL cert warning... its a shared server with just 1 ip address. Are there any workarounds for this warning?
You can use any domain or subdomain for your mailserver. It just is a de facto standard to use mail.domain, imap.domain, smtp.domain, etc. as it's easier and I do believe some (older) software defaults to it. As far as pointing it to the mailserver, either a cname to the fqdn or a record to the ip is just fine. Outlook auto connects tend to look for some special information when automatically setting up a mail address but manually setting it to the right server should work. As for the certificate warning, that has nothing to do with the IP. Just get an SSL certificate (for example from Let's Encrypt) and make sure the mailserver is using it. If you are on a shared hosting account that might be more difficult, but in that case your host should be able to provide you with a hostname that is in the actual certificate.
Think of it as kind of an unofficial naming convention. You can use whatever you want to use for your mail domain but many just stick with mail.domain.tld. I suspect this could help with logging if need be, it would get logged as mail.domain instead of your regular domain which could cause confusion.
As Atlantia Cloud said, you can use any subdomain you like for your servername as long as you can get an SSL certificate for it. In practice, that means you need to own that subdomain. You also need to set the "PTR" DNS record (sometimes called "reverse lookup") for the server IP address to point to that subdomain, otherwise outgoing mail may be rejected by spam filters. That PTR record is usually set by the hosting provider in their control panel.

On the subject of Outlook autoconfiguration: Outlook has changed over the years and the way autoconfiguration works is different for Exchange servers and ordinary IMAP accounts. The current versions of Outlook 365 just use your email address to guess the server settings. If you're using a personal domain like [email protected] then Outlook will first try to connect to an IMAP server called "imap.mydomain.com" and an SMTP server called "smtp.mydomain.com". If your server name is different from your personal domain and the SSL certificate is not signed by both those subdomains, the automatic configuration will fail and users will have to enter the server name manually.

That's fine if your personal domain is the only domain on the server (as it would often be for a business) or if your email addresses use a generic domain like gmail.com rather than a personal domain. But if you're using a personal domain on a shared host this means that the host has to either sign the server certificate with every domain it hosts, or else it has to present different certificates depending on which server name is being asked for. (Dovecot can do the latter, Postfix has to do the former.)

Other mail clients work in different ways, it would take too long to explain them all here. There's no autoconfiguration setup that works with all clients for personal domains that I'm aware of, which is a shame in this day and age.
Phil McKerracher
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Not a hosting company, but I fix hosting problems