What about Tom?

I use tom@hotmail.com as my default bogus email address. Tom has subscribed to so many things because of me.

This is why address verification doesn’t work. The address tom@hotmail.com is a real address. It exists. It accepts mail. And at least one person admits to signing up Tom for mail that the person doesn’t want. I’m sure he’s not the only one.

<- 250 OK
-> MAIL FROM:<test@gmail.com>
<- 250 test@gmail.com....Sender OK
-> RCPT TO:<tom@hotmail.com>
<- 250 tom@hotmail.com
-> QUIT
<- 221 SNT004-MC1F46.hotmail.com Service closing transmission channel

Signups like this do contribute to deliverability problems in a number of ways.

1. The address tom@hotmail.com may be a spam trap. Sending mail to too many spamtraps can result in mail going to the bulk folder.
2. It may belong to an actual person. Sending mail to someone who has not asked for it can result in spam complaints and bulk foldering.
3. It may be a canary address. This is one that gets a lot of spam. If mail goes to this address then it’s likely spam and similar mail is bulk foldered at other recipients.
4. This may be a test address. It might be used by someone inside Hotmail as a test address.

A few of these fake signups aren’t going to hurt deliverability for most senders. Over time, however, these fake signups are going to accumulate on a list. As they accumulate they can, and do, start to affect deliverability. It you’re hitting a lot of these types of addresses it tells the ISP that you don’t have good subscription practices. The ISP knows you are sending mail to people who don’t want it and never asked for it. Filters are designed to affect mail sent to people who don’t want it and never asked for it.
These kinds of addresses are one reason delivery folks talk about segmenting mail based on engagement. Getting rid of even a few of them off a list can improve deliverability for an otherwise opt-in list.
Address verification doesn’t weed out these address. The test I did above is exactly what most of the verification services do. They’re going to return that tom@hotmail.com is a valid address and it’s going to end up in your list.
For those of you who are the type to put real addresses into signup forms, please don’t. It’s unfair to the people who own those addresses and it’s unfair to marketers. Use an address that you know doesn’t exist – like anything@example.com, .net or .org. These domains are reserved by ICANN and will never have users. Stop directing your unwanted email to an innocent 3rd party. They don’t deserve it, and neither do the senders you’re trying to avoid.