A good source of inspiration is Thunderbird with their autoconfig:

All the lookup mechanisms use the email address domain as base for the lookup. For example, for the email address fred@example.com , the lookup is performed as (in this order):

• tb-install-dir/isp/example.com.xml on the harddisk
• check for autoconfig.example.com
• look up of "example.com" in the ISPDB
• look up "MX example.com" in DNS, and for mx1.mail.hoster.com, look up "hoster.com" in the ISPDB
• try to guess (imap.example.com, smtp.example.com etc.)

Dust-Mail already implement good things related to discovery and could be use as a source of inspiration.

Original post:

sender = "smtp" shouldn't make host/post mandatory, these can be auto-discovered with the email address using DNS:

https://www.rfc-editor.org/rfc/rfc6186

• Implement a Thunderbird autoconfig codec (a parser already exists here). (~2d)
• Implement a Microsoft autodiscover codec (a parser already exists here). (~2d)
• Implement a DNS discover, based on dns-mail-discover. (~2d)
• Implement and plug auto configuration of an AccountConfig. This should be done in the lib so it can be used by interfaces. (~2d)
• Redact docs. (~1d)