What is it if anything that happens if my internet cuts out at my home, should I instead use some sort of hosting provider for my mail server?

My ISP has a nice service, called tertiary MX. It's a MX server to add at the lowest priority that will accept emails into a queue that attempts to delivery to the primary MX every 15 minutes for a month.

    > What is it if anything that happens if my internet cuts
    > out at my home,

You bounce any incoming emails in that period, and (obviously) can't send any.

    > should I instead use some sort of hosting provider
    > for my mail server?

I suppose that's a trade-off against how reliable you consider your ISP and electricity supply, and the volume or importance of email you'd be likely to receive in such a period.

Bounce? No. The servers trying to send mail to your server will retry for up to, typically, four or five days, before they bounce the mail back to the sender.

Mail was written to be resilient against network downtime.

That's why you soft bounce. It tells the sender that you can't receive the email _at this time_.

Pretty sure you can't do anything when you're not connected to the internet.

A connection timeout is a soft bounce.

https://tools.ietf.org/html/rfc5321#section-4.5.4.1

   The sender MUST delay retrying a particular destination after one
   attempt has failed.  In general, the retry interval SHOULD be at
   least 30 minutes; however, more sophisticated and variable strategies
   will be beneficial when the SMTP client can determine the reason for
   non-delivery.

   Retries continue until the message is transmitted or the sender gives
   up; the give-up time generally needs to be at least 4-5 days.  It MAY
   be appropriate to set a shorter maximum number of retries for non-
   delivery notifications and equivalent error messages than for
   standard messages.  The parameters to the retry algorithm MUST be
   configurable.

That's why sending mta's should resend. Think sendmail does this every few hours or so

Ha, I wrote that so cautiously, thinking "I'm sure this terminology's not right, I'll probably be called out on it"!

Thanks though, I learned something :)

Regardless of what we call it - the sender would get a copy back with a message not containing the word 'bounce' that says it failed to send, right?

No, no message will be sent until after 4-5 days have passed without connection.

It's usually a bad idea to run email from a home internet connection because it massively increases the chances of your mail being marked as spam. The hardest part of running a mail server is making sure your mail gets through spam filters, and the IP is critical to this.

I forgot to mention that I use the abovementioned setup on a dedicated server, while also setting up a secondary MX dns record for bouncing the mail in case of malfunctioning as sister comments suggested.

I've been doing a similar thing but with Poste.io. https://poste.io/

It has a decent web admin panel built-in to the box and also a copy of RoundCube Webmail. It'll even automatically set-up LetsEncrypt SSL for you.

You might want to set-up rainloop manually though because RoundCube isn't the best.

that's the difference between rainloop and roundcube?