I'd argue having a home mail server is nothing like sending email for a business.

If your business sends out any kind of transactional emails, the Junkie McJunkbuttons of the interwebs will trash your domain and IP reputation because they use the Junk button as the Delete button.

Why does this matter? Please, let me invite you in to this rabbit hole. What I say here is based on information my best friend, who is an engineer working on email transport for a national ISP. And let me tell you, blacklists like Spamhaus, SORBS, etc. and code like SpamAssassin are the Tonka toys of spam filtering.

1. The storage for their mail system is close to 1PB of usable storage via RAID 60 storage modules in their SAN. It's a lot of drives. A lot of Enterprise-class, high RPM, expensive drives.

2. The SAN stores about 3% of all email they receive. This is not because their users are all about Inbox Zero. No, this is because they route ~97% of their received email straight into the bit bucket. Because it is clearly SPAM. Think about that 1PB of storage being multiplied to 33PB just because SPAMmers want to send email. Think about even larger ISP/ESPs that operate at much larger scale and face 97% influx of utter crap.

3. Keeping up with the constantly-evolving threat models used by SPAMmers is too much for just about any engineering team that wants to get some real work done, so they outsource the filtering. These are companies like CloudMark and Symantec to name a couple. They provide APIs where you send an email, and they return an extensive SPAMminess profile for that message. They are the Caterpillar 797F's of SPAM filtering. These companies are used by numerous other companies, so SPAM reported at one customer site aids detection at all customer sites. BTW, they have no page to log in where you can request removal from their blacklist.

4. Since false positives on classifying mail as Junk (the Junk that actually gets to the Junk box) is a huge drain on support resources, they decided to employ two such companies from the Email Filtering Industry. This is because they get different profiles from each company. They glued the two results together in multivariate tests until they found the right formula to significantly reduce the "SPAM False Positive" type of support inquiry.

5. When someone hits the Junk button in their webmail client, the ISP reports that email to both of their Email Filtering partners.

These filtering companies can extrapolate the volume of email your domain sends because they see those emails as they are received by their customers. They can also track how many times a user reports it as SPAM. A few SPAM reports won't hurt your domain/IP reputation, but once you hit a certain threshold, you're blacklisted across all their customers.

So what Mailgun or SendGrid can do for a customer in this ecosphere is use their relationships with the CloudMarks and Symantecs of the world. And gain some forbearance to to de-blacklist customers who are sending legit email, or who suffered an incident and were exploited by SPAMmers. I've witnessed this happening on the ConvertKit Facebook group multiple times. A marketer posts that suddenly their emails aren't reaching their canary accounts, say at Gmail. CK support gets in and says they'll contact their email provider. A day later, problem resolved and said marketer reports their mails being delivered again.

So where does Junkie McJunkbutton come in to all of this? Well, they are usually non-hostile people who don't understand the implications of hitting the Junk Button. That they are essentially reporting a bad faith email event to the "Equifaxes of the email world" about you.

PS Junkie McJunkbutton + going it alone for email transport is what will kill PopcornNotify and similar solutions.