Okay. This is a problem that exists because of English but it’s not something immediately taught in Spanish classes and it could really hurt your chances of getting a job in the field of Spanish if you don’t know it.
When writing a letter in English it’s common to start if off as: Dear ___,
And that’s used for anyone. “Dear” is used for friends, colleagues, potential employers, teachers, the president etc.
In Spanish there are two forms of the word and this is SUPER important.
There’s “Querido/a” and “Estimado/a”
Querido/a means “dear” as in “you’re my buddy and I love you”. It’s also sometimes translated as “beloved” for intimate partners as it’s connected to querer which has a secondary meaning of “to love/be fond of”.
Estimado/a means “honorable/esteemed” as in “you’re someone I don’t want to offend so I’m elevating you by giving you this title because I respect you”.
A good judge of when to use querido/a or estimado/a is if you’re on a first-name basis with this person and they’re not in charge of you querido should be okay. Estimado is for people you’re on a last-name basis with or they’re people that should be respected or people you’ve never met.
If you find this too difficult there’s the reliable “Mr./Mrs.” approach. Note that many of these titles can be abbreviated.
Sr. - Señor | Mr. - Mister
Sra. - Señora | Mrs. - Madame/Ma’am (Typically Sra. is used for married women though it’s used for ladies typically over the age of 18, regardless of marital status today.)
Srta. - Señorita | Miss (Typically used for ladies under 18. It used to be for unmarried women but it became almost insulting to refer to an older woman as Srta. which implied she was a spinster.)
Dr. - Doctor - Used for doctors, medical or PhD
Dra. - Doctora | Dr. (female) - Spanish has a gender attached to doctors so when the doctor, medical or PhD, is a woman it’s Dra.
D. - Don | Mr./Lord - Don is a title of nobility and it implies “lord” although in our mostly non-feudal society, it implies a man who owns a lot of land as the Master of his estate or a very rich man.
Dña - Doña | Madame/Lady - Doña is the female equivalent of a Don, so it implies “Madame” or the “Lady of the house/estate”. It’s used for women who are married to rich men or women who were already rich. It tends to imply a woman of good repute or someone who owns property/businesses.
Doncella - Young Miss - The diminutive of doña. Very archaic except in certain circumstances it’s commonly translated as “maiden” and it’s archaic because it typically is connected to the idea that this girl is a virgin but young daughters of a Don/Doña sometimes carry this title.
Usted / Ustedes - typically shortened to Ud. and Uds. in written speech. It’s a way of saying “you” and “you all” when talking to or about colleagues who deserve respect or hold positions of power.
V o Vd. / Vds. - Is another way of writing Ud. or Uds., but perhaps a bit more formal since it comes from vuestra merced and vuestras mercedes which is an honorable way of addressing someone, having its roots in something like “your grace” or “my lord”
Su/Vuestra señoría - Today it means “your honor” when referring to a judge or a person of high regard; in its archaic setting it means “your lordship” or “your ladyship” and even if it’s a male judge it’s said as vuestra if you’re using it. Most people use su though.
*Prof. / Profesor - Professor - Sometimes used. Not always. Usually people write it out because…
*Prof.ª / Profesora - Never written as “profa”. The little a is sometimes hard to find so people usually write it out as profesora.
*Never write “el/la profe” because that’s slang for teacher. It’s like calling your teacher “teach”.
NEVER use querido/a for your teachers or potential employers.
It makes you look like you’re their best friend. The equivalent is kind of like “Yo, Teach” or “Yo, Boss”. And it’s a good way to show that you’re either not proficient in Spanish enough to be talking or dealing with potential customers/clients/representatives etc. or that you’re way too friendly with people who are not really your friend.