The point as in subject line seems to be buggy (Mail>Preferences>Accounts>"any account defined">Server Settings tab>Outgoing Mail Server group>Account drop-down list). If to select Edit SMTP Server List ... not all defined SMTP servers are listed. All are listed not until one looks at the Account drop-down list in SMTP settings of particular account or uses System Preferences > Internet Accounts.
Is this a known problem?
I have more details now, these explain also the reason of discrepancy in indicated number of list items.
Pop-up window accessible via Edit SMTP Server List ... lists 13 SMTP servers
Account drop-down list in SMTP settings of particular account lists 16 SMTP servers
Those 3 servers not listed on former list are associated with Google accounts - here again point where Google mail services get special handling cause specific in certain sense.
If you are not seeing the iCloud SMTP server listings, I think that is normal.
Mail app SMTP servers listings
