Fix spelling errors

[wneessen]

Description

From GoReportCard:

Misspell Finds commonly misspelled English words

smtp/auth_login.go
Line 32: warning: "challange" is a misspelling of "challenge" (misspell)
Line 32: warning: "compatiblity" is a misspelling of "compatibility" (misspell)

smtp/smtp_test.go
Line 2140: warning: "challanges" is a misspelling of "challenges" (misspell)

auth.go
Line 38: warning: "mechansim" is a misspelling of "mechanism" (misspell)
Line 54: warning: "mechansim" is a misspelling of "mechanism" (misspell)
Line 79: warning: "mechansim" is a misspelling of "mechanism" (misspell)
Line 98: warning: "mechansim" is a misspelling of "mechanism" (misspell)

eml.go
Line 386: warning: "seperately" is a misspelling of "separately" (misspell)

To Reproduce

Check https://goreportcard.com/report/github.com/wneessen/go-mail

Expected behaviour

Use correct spelling

Screenshots

No response

Attempted Fixes

No response

Additional context

No response