Learn Turkish: E-postadaki linke güvenme, bu bir oltalama mesajı. - Do not trust the link in the e-mail, this is a phishing message.

The suffix -ki attaches to the locative case -da to mean "the one that is in". While e-postada just means "in the e-mail", adding -ki turns it into an adjective describing the link, meaning "the link that is in the e-mail".

Why does the word link take the -e suffix in linke?

The verb güvenmek (to trust) always requires the object being trusted to take the dative case, which shows direction towards something. Adding the dative suffix -e to the borrowed word link gives us linke.

Why is there an -ı at the end of mesajı?

This forms a noun compound. Oltalama (phishing) and mesaj (message) are both nouns. To link them into a single concept meaning "phishing message", Turkish adds the possessive compound marker -ı to the second noun, creating oltalama mesajı.

Since this lesson is about the conjunction aksi takdirde (otherwise), how could we use it to add a consequence to this warning?

You would place aksi takdirde after the initial negative command to introduce the alternative consequence. For example, E-postadaki linke güvenme, aksi takdirde hacklenirsin means "Do not trust the link in the e-mail, otherwise you will be hacked."