regex101: PCRE (Legacy) Match Workspace

^start of line

Diagnostic-Code:the literal text Diagnostic-Code: (case sensitive)

\Kresets the starting point of the reported match. Any previously consumed characters are no longer included in the final match

.*any character (except for line terminators), repeated any number of times

Non-Capturing Group(?:\R(?!Diagnostic-Code:).*)*

*repeats the group contents below any number of times:

\Ra newline sequence

Negative Lookahead(?!Diagnostic-Code:)

Diagnostic-Code:the literal text Diagnostic-Code: (case sensitive)

.*any character (except for line terminators), repeated any number of times

Global pattern flags

g modifier: global. Finds all matches instead of stopping after the first

m modifier: multiline. Causes ^ and $ to match the start and end of each line, not only the start and end of the string

Search reference

Regular Expression

^Diagnostic-Code:\K.*(?:\R(?!Diagnostic-Code:).*)*

New

Diagnostic-Code: smtp; 550-5.1.1 The email account that you tried to reach does
    not exist. Please try 550-5.1.1 double-checking the recipient's email
    address for typos or 550-5.1.1 unnecessary spaces. Learn more at 550 5.1.1
    https://support.google.com/mail/?p=NoSuchUser 63si4621095ybi.465 - gsmtp

Diagnostic-Code: smtp; 550-5.1.1 The email account that you tried to reach does
    not exist. Please try 550-5.1.1 double-checking the recipient's email
    address for typos or 550-5.1.1 unnecessary spaces. Learn more at 550 5.1.1
    https://support.google.com/mail/?p=NoSuchUser 63si4621095ybi.465 - gsmtp